Experimental design

• repeated measures design: participants are tested on the same task multiple times, with the same or different conditions. Participants receive all levels of the IV.
• strength of repeated measures design: extraneous variables such as individual differences are eliminated so the IV has a true effect on the DV.
• weaknesses of repeated measures design: demand characteristics (participants may guess what the experimenter wants them to do), practice effects (the more they complete the task, the better they get at it).
• independent groups design: two separate groups of participants take part in an experiment, each group receives one level of the IV.
• strengths of independent groups design: reduces demand characteristics because there's no expectation about what will happen.
• limitations of independent groups design: the researcher cannot control participant variables (abilities and characteristics). More vulnerable to the effect of confounding variables.
• matched pairs design: using two groups but participants are matched based on characteristics that affect the DV (such as IQ or memory span) which must be relevant to the study. Each member of the pair is allocated to a group.
• strength of matched pairs design: it reduces the effect of extraneous variables and characteristics are selected and matched.
• limitations of matched pairs design: matching can be difficult and time consuming, not all factors affecting performance can be measured so some unmeasured factor could still influence results.