Psy 330 McBride Ch. 4
Measurement Issues in Research

Test your knowledge about basic design

I.  Scales of Measurement

Nominal - discrete categories

The categories represent qualitative differences in the variable being measured.

Ordinal - ordering or ranking

Often consists of a series of ranks or verbal labels

The categories form an ordered sequence

Interval - how far apart on a given dimension

Categories are organized sequentially, and all categories are the same size

Arbitrary zero point---zero does NOT mean a complete absence of the attribute being measured (e.g., °F)

Ratio - an interval scale with an absolute zero point

Equal, ordered categories

The value 0 means a complete absence of the variable being measured (e.g., height in inches, weight in lbs.)

II.  Validity

Face validity—Does the measurement technique look like it measures the variable that it claims to measure? 

Construct Validity—Do measurements of a variable behave in exactly the same way as the variable itself? 

III.  Validity and Sources of Bias

How well does the study answer the question it was intended to answer?

**A researcher's ultimate goal: “This is what happened, and this is what it means.”  Any factor that raises doubts about the research results or about the interpretation of the results is a THREAT TO VALIDITY.

      A. Internal Validity

Extent to which other causes are ruled out—extent to which the difference in behavior (the DV) can unambiguously be attributed to the manipulation (the level of the IV received). 

B. External Validity

Extent to which we can generalize the results of a research study to people, settings, times, measures, and characteristics other than those used in that study.

A cold, hard fact:

It is impossible for a study to totally eliminate all threats to validity. Each study is a set of decisions and compromises. Often there's a trade-off between internal and external validity. Therefore, a single study can never "prove" anything on its own. We need to get converging evidence. Also, we must always be critical consumers of research and make our own decisions about validity and quality of the research.

Threats to Validity

        Extraneous variables vs. Confounding variables
Extraneous variable--any variable in a study other than the two variables of interest
Confounding variable--an extraneous variable (usually unmonitored) that is allowed to change systematically along with the two variables being studied...prevents you from establishing a causal link between your two variables of interest

control by holding constant

control by matching across conditions 

control by randomization