Social Research Methods: Qualitative and Quantitative Approaches

EXPERIMENTAL RESEARCH

FIGURE 6 Comparisons of Results, Classical Experimental Design, Weight-Loss Experiments

ENRIQUE’S NATALIE’S

SLIM CLINIC NUTRITION CENTER

Pretest Posttest Pretest Posttest

Experimental 190 (30) 140 (29) Experimental 190 (30) 188 (29)

Control group 189 (30) 189 (30) Control group 192 (29) 189 (28)

SUSAN’S SCIENTIFIC PAULINE’S

DIET PLAN POUNDS OFF

Pretest Posttest Pretest Posttest

Experimental 190 (30) 141 (19) Experimental 190 (30) 158 (30)

Control group 189 (30) 189 (28) Control group 191 (29) 159 (28)

SYMBOLS FOR

CARL’S CALORIE COMPARISON

COUNTERS PURPOSES

Pretest Posttest Pretest Posttest

Experimental 160 (30) 152 (29) Experimental A (A) C (C)

Control group 191 (29) 189 (29) Control group B (B) D (D)

COMPARISONS

A–B C–D A–C B–D (A)–(C) (B)–(D)

Enrique’s 1 49 –50 0 –1 0
Susan’s 1 48 –49 0 –11 0
Carl’s 31 37 –8 –2 –1 0
Natalie’s 2 1 –2 –3 –1 –1
Pauline’s 1 1 –32 –32 0 –1

A–B Do the two groups begin with the same weight? If not, selection bias may be
possibly occurring.

C–D Do the two groups end the same way? If not, the treatment may be
ineffective, or there may be strong history, maturation, diffusion,
or treatment effects.

A–C Did the experimental group change? If not, treatment may be ineffective.

(A)–(C) Did the number of participants in the experimental group or control group
and change? If a large drop occurs, experimental mortality may be a threat to
(B)–(D) internal validity.

INTERPRETATION

Enrique’s: No internal validity threats evident, shows effects of treatment

Susan’s: Experimental mortality threat likely problem

Carl’s: Selection bias likely problem

Natalie’s: No internal validity threat evident, shows no treatment effects

Pauline’s: History, maturation, diffusion of treatment threats are a likely problem

Note:Numbers are average number of pounds. Numbers in parentheses ( ) are number of participants per group.

Random assignment is made to the experimental or control group.