1

Experimental Research ( 實驗法 )

Social Research Methods 2117 & 6501

Fall, 2006December 18-21, 2006

2

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

3

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

4

Experimental Design Logic

• Learn the language of experiments first:– Subjects ( 受試者 )– Treatment (independent variable) and depend

ent variables– Pretest and posttest ( 前測與後測 )– Experimental and control group– Random assignment ( 隨機分配 )

5

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

6

random assignment and random sampling

7

How to randomly assign?

8

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

9

Matching

10

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

11

Types of Experimental Design

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

12

Preexperimental Designs

• One-shot case study ( 單組個案研究 )– A single group of subjects, experimental stimu

lus

• One-group pretest-posttest design ( 單組前後測設計 )– Other factors may cause changes between pr

etest and posttest

• Static-group comparison ( 靜態組間比較 )– Experimental and control groups, but no prete

st

13

Preexperimental Research Designs

14

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

15

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

16

Design Notation

• O = observation of dependent variable

• X = treatment (independent variable)

• R = random assignment

• Symbols in time order: from left to right

• O1 = pretest, O2 = posttest

• See Table 9.2 (p. 261)

17

A Comparison of Various Experimental Design

18

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。 )

19

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 )

20

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 )

21

The double-blind experiment

22

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

23

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

24

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

25

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 )