The data you use should be empirical rather than textual. For this reason, a theory from a secondary source is not evidence, and most professors discourage the use of quotes. However, while psychological data is scientific,
how you interpret it is inherently qualitative. While in the natural sciences, for example, if you want to know the temperature at which a metal melts, you can take a thermometer and watch and see when it melts and you’ll know the answer: you can observe the physical process you want to know about directly. In psychology you can almost never see the psychological process you want to know about because it’s inside a person. For example, you could have an idea that a pharmaceutical increases hunger and then, when you give this pharmaceutical to people they eat more. Did the pharmaceutical increase hunger? Possibly. That’s a good possible explanation for what happened, but you didn’t see hunger, you just saw eating and you have to make a link between what you saw (which was people eating) and the underlying thing you think is in there (which is hunger). In this way, while the data you collect are wholly scientific, the way you choose to explain that evidence is, by nature, conjectural.