There are many different ways in which psychologists can collect information for their study. These are called research methods and it is up to psychologists to choose the one they think is the n1ost appropriate for their study. The main ones are:
experiments (laboratory and field)
self-reports (questionnaires and interviews)
case studies
observations (naturalistic, controlled, participant).
As well as choosing the most appropriate research method, psychologists have to decide:
who the participants are (the sample) and how they are going to recruit (the sampling technique)
if they are using an experiment, which design of study to use (e.g. repeated measures, independent groups or matched pairs)
what the procedure of the study will be for participants including what apparatus is needed
the type of data they want to collect (e.g. quantitative or qualitative).