Imagine measuring someone's height with a tape measure. The numbers and their meaning stay the same no matter who you measure. The intervals on the tape measure are fixed and unchanging. 

But when it comes to measuring mental health symptoms, things get tricky. We usually rely on questionnaires that ask people about their experiences. For instance, if you ask a group of people if they've been feeling less pleasure in activities over the past month (a symptom of depression known as anhedonia), their answers can be influenced by a host of factors. These might include cultural expectations, social environments, whether they’re hungry or tired, and how consistently they feel that way. Rating anhedonia on a scale from 1 to 5 can mean different things to different people, like if the lines on a measuring tape moved closer together or farther apart depending on how someone was feeling. 

Why does this matter? Well, if the meaning of one inch changed depending on who you were measuring, comparing people would be a nightmare. You could end up thinking someone measured at five feet tall is actually taller than someone measured at six feet. This would make it really hard to figure out what factors led to someone being taller or shorter. 

This is what happens in mental health studies. It’s tough to separate a person’s actual depression from what they report on a questionnaire. Our study aims to uncover the factors that lead to these measurement errors in questionnaire responses.