Experimental Studies in Psychology Use Blinding to Prevent Researchers from Biasing

Essay

Experimental studies in psychology use blinding to prevent researchers from biasing their measurements of subjects in the study. In a study of a psychotherapeutic
intervention, a blinded clinician was asked to guess what treatment each subject
received. Data from that experiment are shown below. Suppose a subject is to be
chosen at random from the subjects in this study. $$\begin{array}{l}\text { Frequencies for Blinding Experiment }\\\begin{array} { | l | c | c | } \hline & \begin{array} { c } \text { Correct } \\\text { Guess }\end{array} & \begin{array} { c } \text { Incorrect } \\\text { Guess }\end{array} \\\hline \text { Therapy } & 30 & 6 \\\hline \text { Placebo } & 26 & 11 \\\hline\end{array}\end{array}$$ a) What is the probability that the clinician made a correct guess for the selected
subject?
b) What is the probability the selected subject will have had therapy?
c) What is the probability the selected subject will be one for whom the clinician
correctly guessed had therapy?
d) What is the probability the selected subject will be one for whom the clinician
guessed correctly or who received the placebo treatment?
e) What is the probability the selected subject will be one for whom the clinician
guessed correctly and who received the placebo treatment?
f) In a few sentences, explain why the probabilities calculated in parts (d) and (e)
differ.