Presentation on theme: "Quasi Experiments Non-Experimental Research"— Presentation transcript:
1 Quasi Experiments Non-Experimental Research Research Methods & StatisticsFall 2011
2 Quasi Experiments“Research procedure in which the scientist must select subjects for different conditions from preexisting groups”Research Methods, McBurney & White“An empirical study used to estimate the causal impact of an intervention on its target population. Quasi-experimental research designs share many similarities with the traditional experimental design or randomized controlled trial, but they specifically lack the element of random assignment to treatment or control. Instead, quasi-experimental designs typically allow the researcher to control the assignment to the treatment condition, but using some criterion other than random assignment.Wikipedia, Nov. 17, 2011
3 It’s a matter of control True ExperimentQuasi ExperimentRandom assignment of subjects to conditionManipulate the IVControl allows ruling out of alternative hypothesesSelection of subjects for the conditionsObserve categories of subjectsIf the subject variable is the IV, it’s a quasi experimentDon’t know whether differences are caused by the IV or differences in the subjects
4 Other featuresIn some instances cannot completely control the what, when, where, and howNeed to collect data at a certain time or not at allPractical limitations to data collection, experimental protocol
5 ValidityInternal validity is reduced due to the presence of controlled/confounded variablesBut not necessarily invalidIt’s important for the researcher to evaluate the likelihood that there are alternative hypotheses for observed differencesNeed to convince self and audience of the validity
6 External validityIf the experimental setting more closely replicates the setting of interest, external validity can be higher than a true experiment run in a controlled lab settingOften comes down to what is most important for the research questionControl or ecological validity?
7 Nonequivalent Control Group Designs 2 groups, non-random allocation of subjects and groups, pre-test, treatment (Y/N), post-testDesired pattern:Dependent variables have equal pre-test values, difference seen between experimental and control groups on post-test (Pre: LL, Post: HL)Want to show that any differences that exist did not impact the value of the variable of interest
8 Exercise Interpret the following graphs (experimental, control): Pre: (L, H) Post: (M, H)Pre: (L, M) Post: (H, ML)Pre: (L, M) Post: (H, M)If unequal initial values, it is only if the control remains constant and the experimental goes from being lower than the control to higher than it (the third case), that can you have good confidence that the IV being manipulated was responsible for the changeIn the first case, the control group remained constant and higher than the experimental group. There might be a ceiling effect so that it was not possible for the control group to perfom any better – the improvement in the experimental group might not be due to the experimental manipulationIn the second case, the experimental group was always better than the control group, but both improved for the post-test. Improvement likely caused by maturation.
9 Mixed Factorial Design with One Non-manipulated Variable Example: experiment on pain perception (effect of caffeine, expected differences between men and women)Protocol:25 men/25 women, each takes part in two sessions, one week apartOne session: drink coffee (decaf) and put hand in ice-water until feel painOther session: drink coffee (caffeinated) and put hand in ice-water until feel painBetween subjects variable (male/female)Within subjects variable (caffeine intake)
10 Non-Experimental Research Read and understand:Gabriella Belli - Nonexperimental Quantitative Resarch“Any quantitative study without manipulation of treatments or random assignment is a non-experimental study”Experimental research shows cause and effectNon-experimental research studies variables as they exist
11 Purpose Descriptive: Predictive: Explanatory : primary focus for the research is to describe some phenomenon or to document its characteristics.Predictive:primary focus for the research is to predict some variable of interest (criterion) using information from other variables (predictors).Explanatory :the primary focus for the research is to explain how some phenomenon works or why it operates.
12 Time Frame Cross-sectional: Prospective (longitudinal): Retrospective: data are collected at one point in time, often in order to make comparisons across different types of respondents or participants.Prospective (longitudinal):data are collected on multiple occasions starting with the present and going into the future for comparisons across time.Retrospective:look back in time using existing or available data to explain or explore an existing occurrence.