Experimental Research ( 實驗法 ) Social Research Methods 2117 & 6501 Fall, 2006 December 18-21, 2006 1

Experiments

• Its greatest strength: enabling researchers to testing causal relationships ( 優點 : 可以探究因果關係 ) • conform more to a positivist approach 2

Research Questions Appropriate for Experiments • Well suited for relatively limited and well defined concepts and propositions ( 適合範 圍有限，定義清楚的概念和命題 ) • Appropriate for hypothesis testing, but only for one or a few causal variables • Micro-level questions: individual or small group phenomena 3

Experimental Design Logic

• Learn the language of experiments first: – Subjects ( 受試者 ) – Treatment (independent variable) and dependent variables – Pretest and posttest ( 前測與後測 ) – Experimental and control group – Random assignment ( 隨機分配 ) 4

Selecting Subjects for Experiments: Random Assignment • Random assignment: random in a statistical sense (equal chance of being selected) • Why random assignment?

– To make comparisons – Generalizability ( 概推性 ) – Unbiased • Make experimental and control groups comparable 5

random assignment and random sampling 6

How to randomly assign?

7

Matching (

配對

)

• Similar to quota sampling • Note: the overall average description of the experimental group should be the same as that of the control group • Comparable on what variables? Usually those related to the dependent variable under study 8

Matching

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Matching or Randomization ( 隨機化 )?

• The case for randomization: – Don’t know which variables for the matching.

– Statistics used for analyzing experiment results assume randomization • But with only a few subjects, matching would be better.

• Can combine matching and randomization – Ex: create strata 10

Types of Experimental Design

• Again, components of classical experimental design ( 古典實驗法的三個要素 ) : 11

Preexperimental Designs

• One-shot case study ( 單組個案研究 ) – A single group of subjects, experimental stimulus • One-group pretest-posttest design ( 單組前 後測設計 ) – Other factors may cause changes between pretest and posttest • Static-group comparison ( 靜態組間比較 ) – Experimental and control groups, but no pretest 12

Preexperimental Research Designs 13

Quasi-Experimental Designs: Solomon four group design ( 所羅門四組設計 ) 14

Quasi-Experimental Designs

• • • Posttest-only group design ( 後測控制組設計 ) Contains only group 3 and 4 in the Solomon design Randomly assigning subjects to experimental and control groups Randomization: make subjects comparable on the dependent variable 15

Design Notation

• O = observation of dependent variable • X = treatment (independent variable) • R = random assignment • Symbols in time order: from left to right • O 1 = pretest, O 2 = posttest • See Table 9.2 (p. 261) 16

A Comparison of Various Experimental Design 17

Validity Issues in Experimental Research •

internal validity (

內在效度

)

: the hypothesized independent variable alone affects the dependent variable. ( 以實驗設 計排除自變數已外的變項， 只有自變數影 響了依變數。 ) 18

Threats to internal validity

– Selection bias ( 選擇偏差 ) – History – Maturation – Testing – Instrumentation ( 測量工具 )(related to measurement) – Experimental mortality ( 實驗流失 )(subjects drop out) – Statistical regression (involving extreme cases) – Diffusion of treatment or contamination – Compensatory behavior ( 補償行為 ) – Experimenter expectancy ( 實驗者期望 ) 19

The double-blind experiment (

雙盲實驗

)

Neither the subjects nor the experimenters know which is the experimental group and which is control. ( 受試者與實驗者都不知誰 是對照組或控制組 ) 20

The double-blind experiment

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External validity (

外在效度

)

• External validity: the ability to generalize experimental findings to events and settings outside the experiment itself ( 實驗發現可以概推 到現實世界 ) • Campbell and Stanley: the interaction between the testing situation and the experimental stimulus ( 測驗情境與實驗刺激產生交互作用 ) • Can use Solomon four-group design to address this interaction problem 22

External validity: reactivity ( 實驗反應 ) • Reactivity: subjects are aware of that they are in an experiment and being studied – Hawthorne effect ( 霍桑效應 : 受試者的反應其 實是針對研究者對他們的關注 ) – Demand characteristics – Placebo effect 23

Ethical Concerns in Experimental Research • Experiments are intrusive ( 實驗介入性高， 影響到受試者 ) • Deception ( 欺騙 ): dishonesty is acceptable only as the means to achieve a goal that cannot be achieved otherwise – Debriefing needed 24

Strengths and Weaknesses of Experiments • Strengths – The isolation of the experimental variable and its impact over time – Limited in scope; can replicate • Weaknesses – Its artificiality ( 人為造作、人工化 ) • Overall, its great advantage– logical rigor ( 邏輯嚴謹 ) 25