Soc. 2155 Week 3 Causation and Experiments I. Causation – Relationships between variables – Types of association – Criteria for causality II. Experiments – testing cause and effect – Explanatory research – True experimental designs – Quasi-experimental designs – Internal validity – External validity – Ethical issues – Strengths and weaknesses
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Soc. 2155 Week 3 Causation and Experiments I. Causation –Relationships between variables –Types of association –Criteria for causality II. Experiments.
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Soc. 2155 Week 3 Causation and Experiments
I. Causation– Relationships between variables– Types of association– Criteria for causality
II. Experiments – testing cause and effect– Explanatory research– True experimental designs– Quasi-experimental designs– Internal validity– External validity– Ethical issues– Strengths and weaknesses
Association = relationship• Does not have to be causal.
• Positive association = as X increases, Y increases.
• Negative association = as X increases, Y decreases.
• Qualitative variables: presence of X predicts presence or absence of Y.
Which associations could be causal?
• Years work experience/ income
• # churches / # bars in a town
• Cigarette smoking/ lung cancer
• # firefighters called to fire/ $ amount of damage
• Race/ poverty
Spurious association = apparent association caused by a third factor
Cause = necessary and sufficient condition
Necessary: X must be present in order for Y to follow.
(ex: to get an “A” it is necessary to complete all assignments).
Sufficient: If X occurs, Y must follow.(ex: if you get 100% on every assignment, you will get an
“A” in the class.)
3 criteria for causalityX causes Y if:
• X precedes Y in time
• X and Y are statistically associated
• All other potential causes of Y have been ruled out.
Additional Criteria
• Mechanism – connection between “cause” and “effect” – how the cause operates to produce the effect.
• Context – situations, groups, places, conditions, etc. In which the cause produces the effect.
Determinants/ partial causesMost sociological phenomena have multiple
causes. “Determinant” = partial cause or predictor. Not a complete cause.
Example: Some determinants of income:
Education
Skill
Training
Experience
Intelligence
Marital status
Talent
Personality
Job duties
Type of company
Occupation
Gender
Race
Geographic area
Industry
Types of Causes
Nomothetic Cause – General explanation of a class of phenomena. (e.g., causes of terrorism, crime)
Idiographic Cause – Specific event or sequence of events. (e.g., causes of 9/11 attacks, sudden rise in crime rates) May be historical in focus.
Multivariate Relationships
ZX
Y
Y
Y
Y
Z
Z
Z
XX
X
Multiple causes (determinants) of Y
Z as spurious cause of X and Y
Direct and indirect effects
Z intervenes B/T X and Y OR Z “explains” relationship B/T X and Y
• What is explained? Variation in the dependent variable
• What can be studied in an experiment?Limited, narrow causal relationships
Variables that can be studied in lab
Topics for which theory has been developed
True experiment includes
• Two groups (experimental and control)
• Random assignment to groups
• Variation in independent variable (manipulated by researcher)
• Measurement of dependent variable
The groups
• Experimental group – is exposed to independent variable (I.V.)
• Control group - is not exposed to I.V.
• I.V. is the only difference between the groups
• Any differences in dependent variable (D.V.) must be due to I.V.
Assignment to groups
• Randomization
– Easy to carry out
– Can control for unmeasured or uncontrolled factors
• Matching
– Specific characteristics matched in both groups
– May be very precise
– Requires knowledge of relevant characteristics
– May not control for omitted factors
Pretesting
• Measures D.V. before experiment
• Establishes comparability of experimental and control groups
• Provides baseline for comparison with posttest
• May teach or “clue in” subjects (pretest effect)
• Costs extra
Experimental Designs
Effect of I.V. = (O3-O1) – (O4-O2)
Groups Pretest I.V. Uncontrolled factors
Posttest Change
Exper. O1 X X O3 O3-O1
Control O2 X O4 O4-O2
Classic Pretest-Posttest-Control-Group
Experimental Designs
Effect of I.V. = (O1-O2)
Eliminates effect of pretest
Groups Pretest I.V. Uncontrolled factors
Posttest Change
Exper. N/A X X O1 N/A
Control N/A X O2 N/A
Posttest-Only
Experimental Designs
Effect of I.V. = (O3-O1) – (O4-O2) or (O5-O6)
Effect of pretest = (O3-O5) or (O4-O6)
Groups Pretest I.V. Uncontrolled factors
Posttest Change
Exper. 1 O1 X X O3 O3-O1
Control 1 O2 X O4 O4-O2
Exper. 2 X X O5
Control 2 X O6
Solomon four-group
Quasi-Experimental Designs
• May be used when true experiment isn’t possible
• Usually involve fewer controls– No control group– Approximately equivalent control group– May take place in the field– May be “ex post facto:” designed after the
“treatment”
Internal ValiditySource of Invalidity Solution
History – outside events Control group
Maturation – changes in subjects Control group
Testing – subject may learn Control group
Instrumentation - measurement Control group
Statistical regression - moderation Control group
Selection bias- groups not comparable Randomization