psych 2301-experimental design lecture

basic experiment

-obtain two equivalent groups

-introduce independent variable 

-measure effect of independent variable on the dependent variable 

assigning particpamts

1. independent group designs

2. repeated measures designs

3. matched pairs design

independent group design

-different partipants are assigned to each level of the independent variable

-assignemnt to conditions is random

pretest posttest designs

-sample size is small

-need specific particpants for experiement

-likelihood of dropout is high

disadvantage:

-time consuming

-sensitve to partipants

soloman four group design 

-assess the impact of pretest as a second independent variable 

-half particpants recieve pretest and posttest 

half particpants only recieve posttest 

repeated measures design

same particpants experience all levels of the independent variabele

advantages:

fewer particpants needed

-extremely sensitve detecting measures

-independent variable levels

disadvantages:

-order of presenting treatments may affect dependent variable

-order effect can become the dependent variable

order effects

1. practice effect- particpants doing something multiple times (get better at it)

2. fatigue effect- being tired by time experiencing 2nd level

3. constant effect

solutions to order effects-

-complete or partial counterbalancing

-all possible orders of presentation are included in the experiemnt

partial counterbalancing

-each condition appears at each ordinal postion

-each conditon precedes and follows each conditon one time

-spacing time intervals

-useful to counteract fatigue effect + constrast effect

-drop out rate likely to increase

chosing independent group design or repeated measure design

-both have advantages

-reverse ability of effect

-generziablity of results

method pairds design

-making equal groups

1. match particpants on a key variable

2. randomly assign

   -complex and used less frequently