Counting how often behavior happens is the most common measurement job in ABA. The number you report, though, depends on whether you keep the raw tally, divide it by time, or divide it by opportunities, and each choice tells a different story.
Why it matters
Decisions about a client's program rest on these numbers. If one session lasted 30 minutes and the next lasted 90, a raw count can make behavior look like it tripled when nothing changed. If a learner had only two chances to respond, "100% correct" can look like mastery when it reflects a single good moment. As a BCBA, you design the data system, train staff to use it, and read the graphs, so you need to know what each measure can and cannot show.
Practice two kinds of questions: calculations (convert a count to a rate or a percentage) and judgments (which measure fits this behavior and setting, and what a given number leaves out).
Key ideas
- Count is the raw tally. It answers "how many?" It is only meaningful when you also know how long you observed, so report the observation period with every count. When every observation is the same length, count and rate tell the same story.
- Rate is count divided by observation time. It is expressed per minute, per hour, or per day. Rate lets you compare sessions of different lengths and is the preferred measure for free operants, behaviors the learner can emit at any time without needing a prompt or trial.
- Percentage is a ratio of a count to a total, multiplied by 100. Common forms are percentage of opportunities (correct responses out of trials) and percentage of total responses (correct out of all responses). It works well for restricted operants, where responding depends on opportunities you control, such as discrete trials.
- Every measure hides something. Count hides time. Rate assumes the learner was free to respond throughout the observation. Percentage hides both the count and the time, and it cannot go above 100, so it cannot show gains in fluency once accuracy is perfect.
- Occurrence measures need countable responses. The behavior should have a clear beginning and end, so observers agree on what counts as one instance. Behaviors that vary a lot in length are better captured with duration as well.
Definitions
- Count: the number of times a response occurs during an observation period.
- Rate: count divided by the length of the observation, such as 0.5 responses per minute.
- Frequency: an ambiguous term. Many behavior-analytic texts, including Cooper, Heron, and Heward, treat it as a synonym for rate, while many practitioners use it to mean count. Read the stem carefully.
- Percentage: a proportion expressed as parts per 100, calculated as (number of target responses ÷ total opportunities or total responses) × 100.
- Free operant: a response that can occur at any time and is not limited by trials or prompts.
- Restricted operant: a response whose opportunities are set by someone or something else, such as answering a presented trial.
Examples from a session
- Rate in a clinic. During a 40-minute play session, Nadia, an RBT, tallies 24 spontaneous mands from Eli, age 6. The rate is 24 ÷ 40 = 0.6 mands per minute, or 36 per hour. The next day the session runs 60 minutes and she records 30 mands. That count is higher, but the rate is 0.5 per minute, which is slightly lower.
- Percentage in a classroom. Mr. Haddad runs sight-word trials with Zara, age 9. On Monday she answers 18 of 24 trials correctly (75%). On Tuesday a fire drill cuts the session to 12 trials, and she gets 9 correct (also 75%). Percentage keeps the days comparable even though the number of trials changed.
- Percentage with a small denominator. At home, Sam, age 14, has three chances to put away laundry each week. Going from 1 of 3 (33%) to 2 of 3 (67%) looks like a big jump on a graph, but it is one additional response. Showing the number of opportunities next to the percentage keeps the picture honest.
- Non-example: a count with no observation period. Ms. Lopez, 34, practices greeting coworkers at a supported job site. Her job coach records "5 greetings" but leaves out the shift length, so nobody can tell whether 5 is high or low. A count is not a usable occurrence measure until the observation time is recorded with it.
Common mistakes / exam traps
- Comparing raw counts across sessions of different lengths. Convert to rate first.
- Using rate for trial-based skills. In discrete trials, the instructor sets the pace, so rate mainly reflects how fast trials were delivered. Percentage correct is usually the better accuracy measure.
- Treating 100% as proof of mastery. Percentage cannot show whether responding is fluent, and with few opportunities one response swings the number a lot.
- Assuming "frequency" means the same thing everywhere. Check whether the item treats it as a count or as a rate.
- Counting long, variable behaviors. Two tantrums of 2 and 25 minutes both add 1 to a count. Pair count with duration when length matters.
Exam trap
When a stem gives you different session lengths, divide by time before comparing. When it gives you different numbers of trials, compare percentages, and check the denominator before you trust a big swing.
Exam trap
Percentage of total responses vs. rate of each response: a percentage can stay flat while both responses rise or fall together. When one response should replace another, graph each one as a rate so you can see whether the unwanted response actually decreased.
Quick check
During a 20-minute recess, a playground aide records that Kofi, age 5, starts 8 interactions with peers. Which value correctly describes his rate of initiations?
Related topics
- Measuring time-based dimensions: duration, latency, and IRT (C.4)
- Continuous vs. discontinuous measurement (C.5)
- Measuring efficiency: trials to criterion, cost-benefit, and training time (C.7)
- Choosing a measure that captures the critical dimension within real-world limits (C.9)
- Graphing data: line graphs, bar graphs, and cumulative records (C.10)