Deck 6: Independent Groups Designs

Question 1

Identify and briefly define the three primary types of control used in experiments.

Answer 1

The three primary types of experimental control are: manipulation,holding conditions constant,and balancing.Manipulation refers to controlling an independent variable,that is,by manipulating the levels of an independent variable.Holding conditions constant involves holding a potentially important factor (or factors)constant in an experiment to keep that factor from confounding the intended independent variable.Balancing is the type of control used to control individual differences variables; in the random groups design balancing is usually accomplished by randomly assigning participants to conditions of the experiment.

Question 2

A researcher was interested in whether divorce and remarriage influence the extent to which children are sociable.At a nearby school,the researcher classified 5^th-graders into one of three groups: intact parents (no divorce),divorced (single-parent families),and remarried parents.The researcher interviewed the 5^th-graders and rated their sociability,and also asked them to complete a questionnaire that assessed their level of comfort in different social situations.
Suppose that the researcher finds that the 5^th graders in the divorce (single-parent)group are less sociable than are 5^th graders in the intact and remarried groups.The researcher concludes that divorce and living in a single-parent home causes children to be less sociable,and that remarriage causes improvements in sociability.Do you accept this researcher's conclusions? Why or why not?

Answer 2

This researcher incorrectly made a causal inference about the effects of divorce and remarriage based on a natural groups design.Natural groups designs provide correlational data regarding the relationship between an individual differences variable and a dependent variable; therefore,causal inferences cannot be made.These three groups may differ on a number of other variables (e.g.,income,household stability,time since divorce)that may be related to children's sociability.These variables are potential alternative explanations for the observed group differences,rather than the three groups the researcher formed.

Question 3

A researcher tested whether exposure to images of very thin fashion models causes young women to be dissatisfied with their own body,compared to exposure to athletic body images or neutral (non-body)images.She randomly assigned 120 women from an introductory psychology course to one of three exposure conditions: very thin female images,athletic female images,or neutral images (e.g.,household objects).Each condition had 10 images,projected individually on a large screen.The young women participated in small groups.Each image was displayed for 1 minute,for a total of 10 minutes of exposure.After viewing each image,participants wrote for 30 seconds a description of the image (the participants were led to believe their memory was being tested).After viewing the images,the women completed a questionnaire about satisfaction with their body.Negative scores indicate body dissatisfaction and positive scores indicate satisfaction with their body.The mean scores for each condition were as follows: Suppose the .95 confidence interval (CI)for the very thin condition is -1.75 to -2.50; the CI for the Athletic condition is -.75 to -1.25; and the CI for the neutral condition is 0 to .50.What claim would you make based on the estimates of the population means for the three groups in the experiment based on a comparison of these confidence intervals?

Answer 3

The confidence intervals for the three conditions do not overlap; based on this we can conclude that the population means for the three groups differ.Body dissatisfaction was greatest in the very thin image condition,followed by the athletic image condition.The least amount of body dissatisfaction was in the neutral image condition.

Question 4

A researcher was interested in whether divorce and remarriage influence the extent to which children are sociable.At a nearby school,the researcher classified 5^th-graders into one of three groups: intact parents (no divorce),divorced (single-parent families),and remarried parents.The researcher interviewed the 5^th-graders and rated their sociability,and also asked them to complete a questionnaire that assessed their level of comfort in different social situations.
Describe the independent variable in this study.

Question 5

One major purpose of conducting experiments is to decide whether a treatment or program effectively changes behavior.A second major purpose for doing experiments is to provide

A) definitive answers to theoretical questions.
B) simple and relatively quick ways of testing and revising hypotheses.
C) methods to obtain results that confirm our hypothesis.
D) an empirical test of hypothesis derived from theories.

Question 6

Briefly outline the steps involved in order to make an inference about the effect of an independent variable using null hypothesis significance testing; be sure to indicate the conditions in which an independent variable is judged to be nonsignificant and when it is statistically significant.

Question 7

A researcher tested whether exposure to images of very thin fashion models causes young women to be dissatisfied with their own body,compared to exposure to athletic body images or neutral (non-body)images.She randomly assigned 120 women from an introductory psychology course to one of three exposure conditions: very thin female images,athletic female images,or neutral images (e.g.,household objects).Each condition had 10 images,projected individually on a large screen.The young women participated in small groups.Each image was displayed for 1 minute,for a total of 10 minutes of exposure.After viewing each image,participants wrote for 30 seconds a description of the image (the participants were led to believe their memory was being tested).After viewing the images,the women completed a questionnaire about satisfaction with their body.Negative scores indicate body dissatisfaction and positive scores indicate satisfaction with their body.The mean scores for each condition were as follows: Someone unfamiliar with research methods criticizes the findings by arguing that women who viewed the very thin images probably were more dissatisfied with their bodies even before participating in the experiment.State whether you agree or disagree with this argument and explain your reasoning.

Question 8

When the three requirements for causal inference are met,an experiment is said to be

A) balanced.
B) internally valid.
C) an independent groups design.
D) held constant.

Question 9

A researcher tested whether exposure to images of very thin fashion models causes young women to be dissatisfied with their own body,compared to exposure to athletic body images or neutral (non-body)images.She randomly assigned 120 women from an introductory psychology course to one of three exposure conditions: very thin female images,athletic female images,or neutral images (e.g.,household objects).Each condition had 10 images,projected individually on a large screen.The young women participated in small groups.Each image was displayed for 1 minute,for a total of 10 minutes of exposure.After viewing each image,participants wrote for 30 seconds a description of the image (the participants were led to believe their memory was being tested).After viewing the images,the women completed a questionnaire about satisfaction with their body.Negative scores indicate body dissatisfaction and positive scores indicate satisfaction with their body.The mean scores for each condition were as follows: Describe the dependent variable(s)in this study.

Question 10

When researchers use the multimethod approach they can reach comparable conclusions about a research question after using different methods to study it.Our confidence in these conclusions increases and the conclusions are said to have

A) convergent validity.
B) concurrent validity.
C) multiple validity.
D) correlational validity.

Question 11

Describe how partial replications can be used to establish both experimental reliability and the external validity of research findings.

Question 12

In a study that investigates the effects of two different doses of a drug on memory performance,memory performance represent the ____ variable and doses of the drug represent the ____ variable.

A) correlational; confounding
B) experimental; control
C) dependent; independent
D) independent; dependent

Question 13

A researcher tested whether exposure to images of very thin fashion models causes young women to be dissatisfied with their own body,compared to exposure to athletic body images or neutral (non-body)images.She randomly assigned 120 women from an introductory psychology course to one of three exposure conditions: very thin female images,athletic female images,or neutral images (e.g.,household objects).Each condition had 10 images,projected individually on a large screen.The young women participated in small groups.Each image was displayed for 1 minute,for a total of 10 minutes of exposure.After viewing each image,participants wrote for 30 seconds a description of the image (the participants were led to believe their memory was being tested).After viewing the images,the women completed a questionnaire about satisfaction with their body.Negative scores indicate body dissatisfaction and positive scores indicate satisfaction with their body.The mean scores for each condition were as follows: What type of experimental design did this researcher use?

Question 14

A researcher was interested in whether divorce and remarriage influence the extent to which children are sociable.At a nearby school,the researcher classified 5^th-graders into one of three groups: intact parents (no divorce),divorced (single-parent families),and remarried parents.The researcher interviewed the 5^th-graders and rated their sociability,and also asked them to complete a questionnaire that assessed their level of comfort in different social situations.
Describe the dependent variable(s)in this study.

Question 15

The factors that researchers control or manipulate in order to determine their effect on behavior are called the

A) intervention variables.
B) dependent variables.
C) independent variables.
D) confounding variables.

Question 16

Explain how the procedures for conducting a matched groups design and a natural groups design differ from the random groups design.

Question 17

Clearly and concisely describe how a researcher could minimize the likelihood of selective subject loss in an experiment-be sure to include in your answer any risks that the researcher would be taking in trying to prevent the selective loss.

Question 18

A researcher tested whether exposure to images of very thin fashion models causes young women to be dissatisfied with their own body,compared to exposure to athletic body images or neutral (non-body)images.She randomly assigned 120 women from an introductory psychology course to one of three exposure conditions: very thin female images,athletic female images,or neutral images (e.g.,household objects).Each condition had 10 images,projected individually on a large screen.The young women participated in small groups.Each image was displayed for 1 minute,for a total of 10 minutes of exposure.After viewing each image,participants wrote for 30 seconds a description of the image (the participants were led to believe their memory was being tested).After viewing the images,the women completed a questionnaire about satisfaction with their body.Negative scores indicate body dissatisfaction and positive scores indicate satisfaction with their body.The mean scores for each condition were as follows: Describe the independent variable in this study.

Question 19

Describe briefly how confidence intervals can be used to determine if the means for the two conditions of an experiment differ.

Question 20

A researcher was interested in whether divorce and remarriage influence the extent to which children are sociable.At a nearby school,the researcher classified 5^th-graders into one of three groups: intact parents (no divorce),divorced (single-parent families),and remarried parents.The researcher interviewed the 5^th-graders and rated their sociability,and also asked them to complete a questionnaire that assessed their level of comfort in different social situations.
What type of experimental design did this researcher use?

Question 21

The most common solution to the problem of forming comparable groups in the random groups design is

A) random-digit dialing.
B) random assignment.
C) random selection.
D) matching participants on the dependent variable.

Question 22

A researcher has manipulated only one independent variable at two levels,has held constant as many other variables as possible and has balanced individual differences by using random assignment.The researcher is likely to be able to claim that the independent variable caused the observed changes in the dependent variable because the experiment is

A) externally valid.
B) internally valid.
C) a natural groups design.
D) statistical.

Question 23

Two control techniques that allow researchers to rule out alternative explanations for an outcome are balancing and

A) holding conditions constant.
B) establishing a time-order relationship.
C) validation.
D) establishing a covariation.

Question 24

The goal of a random groups design experiment is to establish the independent variable as the cause of any differences in the dependent variable.The logic of accomplishing this involves

A) beginning with comparable groups, treating them differently, and ending with differences among groups.
B) beginning with comparable groups, treating them the same, and ending with no differences among groups.
C) beginning with noncomparable groups, treating them differently, and ending with differences among groups.
D) beginning with noncomparable groups, treating them the same, and ending with no differences among groups.

Question 25

Researchers manipulated exposure to body images by randomly assigning young girls to one of three conditions.They observed that young girls' body dissatisfaction was greatest after viewing very thin images of a Barbie doll compared to normal-weight images ("Emme" doll)or neutral images.The alternative explanation that girls in the Barbie condition might have weighed more prior to exposure to body images can be ruled out because the researchers

A) held constant the amount of time girls looked at images.
B) manipulated the type of exposure with three conditions.
C) randomly assigned girls to the different exposure conditions.
D) all of these

Question 26

Individual differences among participants in a random groups design are controlled by

A) eliminating the individual differences from the experiment.
B) ignoring the individual differences because they cannot confound the experiment.
C) balancing the individual differences across the conditions of the experiment.
D) holding the individual differences constant in the experiment.

Question 27

The procedure for using block randomization in an independent groups experiment with three conditions (A,B,C)and 60 subjects is to

A)generate twenty random orders of the conditions (e.g.,ACB,BAC)and assign subjects one block at a time (i.e.,3 subjects in the first block,3 in the second block,and so on).
B)generate one random order of conditions (e.g.,ACB)and test the first 20 subjects in condition A,the second 20 subjects in condition C,and the last 20 subjects in condition B.
C)select intact groups of 20 subjects each and randomly assign the groups to condition A,B,or C.
D)generate sixty random orders of the conditions,one for each subject.

Question 28

A researcher interested in young girls' exposure to thin images of Barbie dolls compared to normal-weight images ("Emme" doll)wanted to make sure girls in the two conditions (Barbie,Emme)did not differ in the number of Barbie dolls they own at home.The best way to do this would be to

A) hold conditions constant by making sure each girl in the experiment only had three Barbie dolls.
B) randomly assign girls to the two conditions of the experiment.
C) give each girl in the experiment a Barbie doll and an Emme doll.
D) none of these

Question 29

A researcher examined participants' memory following emotionally shocking events by manipulating whether participants viewed a violent or a nonviolent version of a video.The video in the two conditions was identical except for two seconds in which the violence variable was manipulated.That the video was identical except for the manipulation represents the control technique of

A) balancing the individual differences across the groups of the experiment.
B) holding conditions constant in the experiment.
C) a time-order relationship between the independent and dependent variables.
D) confounding the independent variable.

Question 30

Which of the following arises when the independent variable of interest and an unintended independent variable are allowed to covary?

A) contamination effect
B) decrease in external validity
C) illusory correlation
D) confounding

Question 31

An instructor randomly assigns two sections of his course to an experimental condition (new teaching method)or control condition (regular teaching method).Students in the 9:30 section receive the experimental treatment and students in the 1:30 section participate in the control condition.At the end of the semester the instructor's tests reveal that students in the 9:30 section had significantly higher test scores than students in the 1:30 section.Based on this summary,we can state that

A) the results are uninterpretable because of intact groups.
B) the findings have external validity across two times of day.
C) the experiment has internal validity.
D) all of these

Question 32

In addition to creating groups of equal size,an advantage of block randomization is that it

A) decreases the number of participants needed in each condition.
B) averages the effect of the dependent variable across the conditions of the experiment.
C) holds constant the time in which an experiment is run.
D) balances potential confoundings that occur during the time in which an experiment is run.

Question 33

A researcher randomly assigns one classroom to a new teaching method and a second classroom to the control condition (the regular teaching method).This researcher faces the potential problem of confounding due to

A) experimenter effects.
B) selective subject loss.
C) intact groups.
D) extraneous variables.

Question 34

In order to conduct an experiment more efficiently,a researcher tests groups of participants in several small groups.The number of participants in each group differs because of participants' availability at different times.The different number of participants in each group represents a potential

A) intact group problem.
B) selective subject loss problem.
C) experimenter effect.
D) extraneous variable problem.

Question 35

Subject loss (attrition)poses a problem for a random groups design because

A) extraneous variables are more likely to affect subjects who drop out.
B) group equivalence established at the beginning of the experiment may be lost.
C) participants may respond to demand characteristics.
D) participants may change their natural group designation.

Question 36

When participants begin an experiment but fail to complete it,the internal validity of the experiment can be threatened.Which of the following types of subject loss poses the most serious threat to internal validity?

A) the loss occurs because of an error by the experimenter.
B) the loss leads to different numbers of participants in the groups in the experiment.
C) the loss occurs because of equipment failure.
D) the loss occurs because of some characteristic of the participant that is related to the outcome of the study.

Question 37

If participants know they have been given alcohol in an experiment,they may expect certain effects such as giddiness or relaxation.The general term for such cues that guide participants' behavior in a study is

A) double-blind effects.
B) self-inflicted characteristics.
C) expectation effects.
D) demand characteristics.

Question 38

One preventive step researchers can take when they suspect selective subject loss might occur in their experiment involves using a pretest to screen out participants who may drop out of the study.The disadvantage of this method is decreased

A) external validity.
B) internal validity.
C) statistical significance.
D) all of these

Question 39

Random assignment to conditions works to balance participants' individual differences across conditions of the experiment by

A) matching individuals on key variables.
B) asking individuals to participate in each condition of the experiment.
C) forming groups based on an individual differences variable the researcher selects.
D) generating groups of participants that are equivalent, on average.

Question 40

Four students plan to conduct an experiment with four conditions (A,B,C,and D)during one semester.Each student will serve as an experimenter.The best way to control for the extraneous variable of having four experimenters test participants in the study is to

A) make sure each experimenter tests only one of the conditions.
B) have the first experimenter test a random order of conditions during the first two weeks of the semester, the second experimenter test a second random order during the next two weeks, and so on.
C) have each experimenter test randomized blocks of conditions.
D) have each experimenter test condition A during the first two weeks of the semester, condition B during the next two weeks, and so on.

Question 41

We can be confident that the population means differ for two conditions of an experiment when the confidence intervals for the two sample means

A) are of different size.
B) have different standard deviations.
C) overlap.
D) do not overlap.

Question 42

Which of the following is a measure of the strength of the relationship between the independent and dependent variables that is independent of sample size?

A) t-test
B) standard deviation
C) mean difference score
D) effect size

Question 43

The nonsystematic (random)variation due to differences among subjects within each group is called

A) spurious variation.
B) nonsignificant variation.
C) inferential variation.
D) error variation.

Question 44

Which of the following is the best way to determine whether the differences in means obtained in an experiment are reliable?

A) Replicate the experiment.
B) Use confidence intervals to compare the means.
C) Perform an inferential statistics test (null hypothesis significance testing).
D) Find the difference between two sample means.

Question 45

.The researcher should

A) reduce the alpha level of significance to .025 to form a definite conclusion.
B) accept the null hypothesis of no difference between the experimental and control groups because the probability is so small; thus, the independent variable had no effect.
C) reject the null hypothesis of no difference between the experimental and control groups because the probability is so small; thus, the independent variable had a reliable effect on the dependent variable.
D) neither accept nor reject the null hypothesis of no difference because the probability value of .025 is significantly different than .05.

Question 46

The statistical technique that is used to analyze the results of several independent experiments is called

A) F-test.
B) central tendency.
C) meta-analysis.
D) Cohen's d analysis.

Question 47

Placebo control groups and double-blind procedures are typically used to control for

A) individual differences variables and matching variables.
B) demand characteristics and experimenter effects.
C) selective subject loss and mechanical subject loss.
D) manipulated variables and extraneous variables.

Question 48

A statistically significant outcome is an outcome that

A) has a large likelihood of occurring if the null hypothesis is true.
B) has a small likelihood of occurring if the null hypothesis is true.
C) always has a large effect size.
D) has neither Type I nor Type II errors.

Question 49

The problem of Type I and Type II errors occurs because

A) decision making based on inferential statistics depends on probabilities.
B) researchers rarely conduct internally valid experiments.
C) the null hypothesis is difficult to define.
D) all of these

Question 50

The three steps of data analysis are

A) find any outliers, compute confidence intervals, and do null hypothesis significance testing.
B) check the data, summarize the data, and confirm what the data reveal.
C) find Type I and Type II errors, compute means, and compute inferential statistics.
D) calculate the standard deviation, find the effect size, and do null hypothesis significance testing.

Question 51

When we conclude that an experiment does not provide sufficient evidence to reject the null hypothesis when,in fact,an independent variable does produce an effect,the inferential statistics problem is called a

A) Type I error.
B) Type II error.
C) no-confidence interval.
D) probability error.

Question 52

After checking the data for errors and outliers,the next step in analyzing the data from a research study is to use

A) confidence intervals.
B) meta-analysis.
C) descriptive statistics.
D) inferential statistics and null hypothesis significance testing.

Question 53

A researcher conducted an experiment in which participants played either a violent or a nonviolent video game.After the game,the researcher measured hostile cognitions.The effect size,Cohen's d,for the difference in mean hostile cognition between the violent and nonviolent conditions was .83.Based on this,it is possible to state that the video game independent variable had a _______ effect on hostile cognition in this experiment.

A) nonsignificant
B) small
C) medium
D) large

Question 54

Which of the following is not an advantage of using measures of effect size?

A) Measures of effect size provide information about the strength of the relationship between an independent variable and a dependent variable.
B) Measures of effect size provide the best information about whether the effect of the independent variable is statistically significant.
C) Measures of effect size can provide an estimate of the overall effect size for an independent variable by averaging effect sizes across a series of experiments.
D) Measures of effect size can be used to make quantitative comparisons of the different outcomes found in a series of experiments involving the same independent variable.

Question 55

Two statistical methods that researchers use to determine whether an independent variable has a reliable effect on a dependent variable are

A) null hypothesis significance testing and confidence intervals.
B) statistical significance and odds-likelihood analysis.
C) analysis of error variation and effect size.
D) Cohen's d and alpha.

Question 56

A statistically significant outcome is an outcome that

A) leads us to reject the null hypothesis.
B) assures the internal validity of the experiment.
C) assures that practical applications of the outcome will be successful.
D) all of these

Question 57

Our ability to draw appropriate conclusions based on the results of an experiment depends most of all on the

A) external validity of the experiment.
B) internal validity of the findings.
C) amount of variation in the experiment.
D) statistical significance of the findings.

Question 58

A Type I error occurs when the null hypothesis is really true and we claim

A) the independent variable did not have an effect on behavior.
B) the independent variable did have an effect on behavior.
C) the inferential test was not statistically significant.
D) that the results are not statistically significant.

Question 59

A researcher computes a .95 confidence interval for an experimental group to be 3.0 B 7.0,and computes a .95 confidence interval for a control group to be 0.0 B 4.0.Based on these confidence intervals,the researcher can state that

A) the results for the effect of the independent variable on the dependent variable are inconclusive.
B) the independent variable definitely influenced participants' scores on the dependent variable because the confidence interval for the experimental group is higher than that that of the control group.
C) the independent variable definitely did not affect participants' scores because the confidence interval for the control group includes zero.
D) the population value for the experimental group is .95.

Question 60

When observers have biases regarding what they expect participants' behavior to be like in an experiment,the observations may be influenced by

A) placebos.
B) demand characteristics.
C) experimenter effects.
D) selective subject loss.

Question 61

When the findings of experiments testing the effectiveness and safety of drugs on animals are replicated in clinical trials with people,the original findings of the animal research are shown to have

A) external validity.
B) internal validity.
C) statistical significance.
D) sensitivity.

Question 62

Which of the following is an example of an individual differences (subject)variable?

A) individuals randomly assigned to one of three different treatments in a diabetes study
B) an independent variable involving three levels of nicotine in a smoking cessation program
C) a law enforcement training program compared over a 4-week or an 8-week period
D) individuals classified as introverted or extraverted in a study of alcohol use

Question 63

When conducting an experiment using the matched groups design,the preferred matching variable is

A) completely different from the dependent variable.
B) an inexpensive test that participants could complete quickly.
C) a measure with limited reliability and validity.
D) the same measure that will be used as the dependent variable.

Question 64

Researchers found a relationship between insults and aggressive behavior for 5-year olds.If they want to test the external validity of the conceptual relationship between insults and aggressive behavior for a sample of 35-year olds,they should

A) test the findings in the real world rather than in a laboratory setting.
B) use the same measure of aggression as was used with the 5-year olds.
C) use age-appropriate insults and measures of aggression for the 35-year olds.
D) use the same insults as was used with the 5-year olds.

Question 65

Which of the following conditions would lead you to recommend against the use of a matched groups design and in favor of a random groups design?

A) a small number of participants from a heterogeneous population is available
B) a large number of participants from a homogeneous population is available
C) a separate group is required for each level of the independent variable of interest
D) a reliable and valid matching variable is available

Question 66

To differentiate experiments involving individual differences (subject)variables and those involving manipulated independent variables,those experiments involving groups selected based on individual differences (subject)variables are called

A) selected subjects designs.
B) matched groups designs.
C) random groups designs.
D) natural groups designs.

Question 67

Which of the following is the most critical problem in drawing causal inferences based on the natural groups design?

A) establishing covariation
B) correlating participants' characteristics and their performance
C) eliminating plausible alternative causes for the obtained relationship
D) using the natural groups variable to make predictions for the dependent variable

Question 68

A researcher plans to use a matched groups design to assess a treatment for increasing body weight of premature infants.The independent variable has two conditions,treatment and control.Eight pairs of premature infants are matched according to their body weight prior to manipulating the independent variable.In order to control for other potentially relevant characteristics of the participants,the researcher should

A) identify a matching variable uncorrelated with body weight.
B) assign the lower weight infant in each pair to the treatment condition.
C) randomly assign infants in each pair to the treatment or control group.
D) none of these

Question 69

In what situation is it argued that external validity is irrelevant?

A) when psychologists seek to describe real-world settings based on laboratory research.
B) when research is conducted with college student samples.
C) when the goal of an experiment is to observe the most typical case found in nature.
D) when the purpose of the experiment is to test a specific hypothesis derived from a theory.

Question 70

The natural groups design represents an illustration of the general research approach that is called

A) correlational research.
B) descriptive research.
C) experimental research.
D) observational research.

Question 71

A researcher read a research report indicating that a certain medication was found to be effective when tested on men.The researcher plans to do an experiment testing the effectiveness of the same medication,but in his experiment both men and women will be tested.The researcher is planning to do a

A) partial replication of the experiment to test the reliability and external validity of the finding from the original experiment.
B) replication of the experiment to test the internal validity of the original experiment.
C) replication of the experiment to test the sensitivity of the original experiment.
D) partial replication of the experiment to test the statistical power of the original experiment.

Question 72

________ refers to the researcher's ability to make causal inferences regarding an experimental outcome and _________ refers to the researcher's ability to generalize the findings beyond the scope of the specific experiment.

A) Reliability; validity
B) Replication; confounding
C) Internal validity; external validity
D) External validity; internal validity

Question 73

A cognitive psychologist wants to do an experiment to test a variable that may influence the memory of elderly people.She has the following constraints: only 12 people are available as possible participants,and the independent variable she plans to manipulate requires the use of a separate group for each of the two conditions.The psychologist has a reliable and valid pretest she can use to evaluate participants' memory prior to the manipulation.Which design is this psychologist likely to use?

A) random groups design
B) natural groups design
C) matched groups design
D) placebo control design