Alright kiddo, strap in, because we're about to dive into the depths of psychology research! Don't fret, we'll make this as fun as possible.

Think of 'validity' like an invisible friend. You can't see or touch it, but you know it's there, or at least, you hope it is. The thing about validity is, we can't measure it directly, but we can play detective and try to figure it out.

How do we do this? We look for things that shouldn't be there – biases. Let's say you've made cookies, and you want to know if you've followed the recipe correctly. You can't just ask the cookies, but if you bite into one and taste curry, you know something's off. In diagnosis, if we find something that shouldn't be affecting the result (like curry in your cookies), we know that the diagnosis might not be as valid as we thought.

Here, we put on our detective hats. There are two main methods that researchers use to investigate diagnosis: