Imagine the horizontal axis is dose of a drug, and the vertical is the response, like hours of sleep. Looking at Lisa's response, it's clear that increasing the dose reduces sleep. Same for Bart. But if you do a linear regression of all the data, shown by the red line, dose increases sleep, which is wrong.
Imagine the horizontal axis is dose of a drug, and the vertical is the response, like hours of sleep. Looking at Lisa's response, it's clear that increasing the dose reduces sleep. Same for Bart. But if you do a linear regression of all the data, shown by the red line, dose increases sleep, which is wrong.