Summary: A line graph tells you three things: which way the data is heading (trend), how high or low it's sitting (level), and how much it's bouncing around (variability). Your job as an RBT is to look at a graph and describe what you see in plain terms, "it's trending up," "it's flat and noisy", not to decide what to do about it. That decision belongs to the BCBA. This page teaches you to eyeball a graph honestly, recognize when a trend is therapeutic, and know exactly where your lane ends.
You're going to spend a lot of your career making graphs without ever really reading them. You tally, you log, the software draws the line, and you move on to the next program. That's fine, until your supervisor points at a graph during a meeting and asks, "what do you see here?" The RBT who can answer that cleanly looks like they know what they're doing, because they do. And the third-edition Test Content Outline now expects you to. Describing what a graph shows is a task on the list, so it's fair game for the exam.
The good news: you don't need statistics. You need to look at a line and say three honest things about it.
The three things every graph tells you
Almost every graph you'll meet in this field is a line graph. Behavior goes up the side (the vertical y-axis), time goes across the bottom (the horizontal x-axis), usually one dot per session or per day. Connect the dots and you've got a line that moves. Reading it means answering three questions.
Trend: which way is it going?
That's the free preview of Identifying Trends in Graphed Data. The complete guide, plus all 851 practice questions and 3 full exams, is in the study pack and the book.