C.3: Measure occurrence.
Free practice questions for this part of the BCBA, with an explanation for every option.
An RBT records a client's vocal stereotypy. On Monday she observes for 30 minutes and counts 18 responses. On Tuesday she observes for 20 minutes and counts 12 responses. Which statement is accurate?
- A. Count is the best measure here because both sessions were longer than 15 minutes.Incorrect. Whenever observation times vary, rate is the right measure, whatever the session length.
- B. The rate was 0.6 responses per minute on both days, so stereotypy did not change.Correct. 18/30 = 0.6 and 12/20 = 0.6 responses per minute. The rate stayed the same.
- C. The rate was 1.67 responses per minute on both days.Incorrect. This is minutes per response (30/18), the inverse of rate, not responses per minute.
- D. Stereotypy decreased from Monday to Tuesday, because the count fell from 18 to 12.Incorrect. When observation periods differ in length, raw counts can't be compared. The lower count reflects the shorter session.
A supervisor notices that one RBT's session lengths range from 15 to 90 minutes, depending on the family's schedule. The RBT reports 'number of requests made per session.' What should the supervisor recommend?
- A. Keep reporting count, because count is the most basic measure of occurrence.Incorrect. Count is only comparable when observation times are constant, which they are not here.
- B. Report the percentage of sessions in which at least one request occurred.Incorrect. This throws away nearly all the information about how often requests occurred.
- C. Convert the data to rate (requests per minute or per hour) so sessions can be compared.Correct. Rate divides count by observation time, which allows a meaningful comparison across sessions of different lengths.
- D. Report the duration of each session instead of the number of requests.Incorrect. Session duration says nothing about how often the target behavior occurred.
During discrete-trial teaching, a learner answers 4 of 5 trials correctly on Monday and 40 of 50 trials correctly on Wednesday. The graph shows 80% correct both days. What is the most important limitation of reporting only percentage?
- A. Percentage always overestimates performance compared with rate.Incorrect. Percentage has no built-in bias toward overestimating. It is a ratio with different limitations than rate.
- B. Percentage is a temporal measure and should be replaced with latency.Incorrect. Percentage is a derived measure of occurrence, not a temporal measure.
- C. Percentage hides the number of opportunities, and a 1-trial change moves the Monday value by 20 percentage points.Correct. Percentages hide the denominator. With few opportunities, each response changes the percentage a lot, so the number of trials should be reported too.
- D. Percentage cannot be calculated for discrete-trial data.Incorrect. Percentage correct is commonly and properly used for trial-based data.