Child Development 133 
Research Methods in Human Development

Sections 04, 68, & 69
Hembree           Spring, 2011


Experimental Methods

I. Structure of an experiment

  A. What is an experiment? Experimental vs. Correlational studies



B.   Independent and dependent variables

1. types of manipulation




2. experimental and control groups




3. Manipulating the Independent variable

  • manipulation check



  • pilot testing




C. Experimental Control and explaining variance in our dependent variable 

1.  sources of error variance 





2. confounds





II. Internal Validity in experiments  

  • Have I reduced error and the effects of extraneous variables?  

 A. Internal vs. External Validity





B. Threats to internal validity

1) history



2) maturation



3) testing



4) instrumentation



5) statistical regression  



6) selection



7) mortality



8) diffusion




C. Discussion: What are some of the ways to reduce these threats?

1)  You want to conduct a study comparing the cognitive abilities of 20-year-olds and 50-year-olds.  You administer a memory test to the two groups.  What threat(s) to internal validity are present?

2) You conduct a longitudinal study of memory skill.  You follow a group of 20-year-old college students, testing them every 10 years. What threats to internal (and external) validity exist?  


III. Controlling unwanted variance

A.  Participant selection/assignment

1.  random sampling




2. random assignment




B. Control within the experiment

1. elimination of unwanted variables



2. constancy



3. balancing



4.  counterbalancing



C. Experimenter effects:

1. experimenter characteristics



2. experimenter expectancies



How to control?



D. Participant characteristics

1. demand characteristics



2. response bias - yea saying and nay saying



How to control?




E. Placebo effects and control groups





            How to control?





Send problems, comments or suggestions to:

California State University, Sacramento
College of Education
Department of Child Development

Updated: January, 2011