Presentation on theme: "Comparing Results from RCTs and Quasi-Experiments that share the same Intervention Group Thomas D. Cook Northwestern University."— Presentation transcript:
Comparing Results from RCTs and Quasi-Experiments that share the same Intervention Group Thomas D. Cook Northwestern University
Why RCTs are to be preferred Statistical theory re expectations Relative advantage over other bias-free methods-- e.g., regression-discontinuity (RDD) and instrumental variables (IV) Ad hoc theory and research on implementation Privileged credibility in science and policy Claim that non-exp. alternatives routinely fail to produce similar causal estimates
Dissimilar Estimates Come from empirical studies comparing exp. and non-exp. results on same topic Strongest are within-study comparisons These take an experiment, throw out the control group, and substitute a non- equivalent comparison group Given the intervention group is a constant, this is a test of the different control groups
Within-Study Comparison Lit. 20 studies, mostly in job training. Of the 14 in job training reviews contend: (1) no study produces a clearly similar causal estimate, including Deheija & Wahba (2) Some design and analysis features associated with less bias, but still bias (3) the average of the experiments is not different from the average of the non-experiments--but be careful here and note the variance of the effect sizes differs by design type
Brief History of Literature on Within Study Comparisons LaLonde; Fraker & Maynard 12 subsequent studies in job training Extension to examples in education in USA and social welfare in Mexico, never yet reviewed
Policy Consequences Department of Labor, as early as 1985 Health and Human Services, job training and beyond National Academy of Sciences Institute of Educational Sciences Do within-study comparisons deserve all this?
We will: Deconstruct „non-experiment“ and compare experimental estimates to 1. Regression-discontinuity estimates 2. Estimates from difference-of-differences (fixed effects) design Ask: Is general conclusion about the inadequacy of non-experiments true across at least these different kinds of non-experiment
Criteria of Good Within-Study Comparison Design 1. Variation in mode of assignment--random or not 2.No third variables correlated with both assignment and outcome--e.g., measurement 3. Randomized experiment properly executed 4. Quasi-experiment good instance of “type” 5. Both design types estimate the same causal entity- -e.g, LATE in regression-discontinuity 6. Acceptable criteria of correspondence between design types--ESs seem similar; not formally differ; stat significance patterns not differ, etc.
Experiments vs. Regression- Discontinuity Design Studies
Three Known within-Study Comparisons of Exp and R-D Aiken, West et al (1998)- R-D study; experiment; LATE; analysis; results Buddelmeyer & Skoufias (2003)-R-D study; experiment; LATE; analysis; results Black, Galdo & Smith (2005)-R-D study; experiment; LATE; analysis; results
Comments on R-D vs Exp. Cumulative correspondence demonstrated over three cases Is this theoretically trivial, though? Is it pragmatically significant, given variation in implementation in both the experiment and R-D? As “existence proof”, it belies over-generalized argument that non-experiments don’t work As practical issue, does it mean we should support RDD when treatments are assigned by need, merit. Emboldens to deconstruct non-experiment further
Experiment vs Differences-in- Differences Most frequent non-experimental design by far across many fields of study Also modal in within-study comparisons in job training, and so it provides major basis for past opinion that non-experiments are routinely biased We review: 3 studies with comparable estimates 14 job training studies with dissimilar estimates 2 education examples with dissimilar estimates
Bloom et al Bloom et al (2002; 2005)--job training the topic Experiment 11 sites - 8 pre earning waves; 20 post Non-Experiment = 5 within-state comparisons; 4 within-city; all comparison Ss enrolled in welfare We present only control/comparison contrast because treatment time series is a constant
Issue is: Is there overall difference between control groups randomly or non-randomly formed? If yes, can statistical controls—OLS, IV (incl. Heckman models), propensity scores, random growth models—eliminate this difference? Tested 1O modes, but only one longitudinal Why we treat this as d-in-d rather than ITS
Bloom et al. Results
Bloom et al. Results (continued)
Implications of Bloom et al Averaging across the 4 within-city sites showed no difference-also true if 5th between-city site added Selecting within-study comparisons obviated the need for statistical adjustments for non- equivalence--design alone did it. Bloom et al tested differential effects of statistical adjustments in between-state comparisons where there were large differences None worked, or did better than OLS
Aiken et al (1998) Revisited The experiment. Remember that sample was selected on narrow range of test score values Quasi-Experiment--sample selection limited to students who register late or cannot be found in summer but who score in the same range as the experiment No differences between experiment and non- experiment on test scores or pretest writing tests Measurement identical in experiment and non-exp
Results for Aiken et al Writing standardized test =.59 and.57 - sig Rated essay =.06 and.16 – ns High degree of comparability in statistical test results and effect size estimates
Implications of Aiken et al Like Bloom et al, careful selection of sample gets close correspondence on important observables. Little need for stat adjustment for non- equivalence limited only to unobservables Statistical adjustment minor compared to use of sampling design to construct initial correspondence
What happens if there is an initial selection difference? Shadish, Luellen & Clark (2006)
Pretests, and then Random Assignment to N = 445 Undergraduate Psychology Students Randomized Experiment n = 235 Randomly Assigned to Nonrandomized Experiment n = 210 Self-Selected into Mathematics Training n = 119 Mathematics Training n = 79 Vocabulary Training n = 131 Vocabulary Training n = 116 All participants measured on both mathematics and vocabulary outcomes Figure 1: Design of Shadish et al. (2006)
What’s special in Shadish et al Variation in mode of assignment Hold constant most other factors thru first RA--population/measures /activity patterns Good experiment? Pretests; short-term and attrition; no chance for contamination. Good quasi-experiment? - selection process; quality of measurement; analysis and role of Rosenbaum
Results Shadish et al
Implications of Shadish et al Here the sampling design produced non- equivalent groups on observables, unlike Bloom Here the statistical adjustments worked when computed as propensity scores However, big overlap in experimental and non- experimental scores due to first stage random assignment, making propensity scores more valid Extensive, unusually valid measurement of a relatively simple selection process, though not homogeneous.
Limitations to Shadish et al What about more complex settings? What about more complex selection processes? What about OLS and other analyses? This is not a unique test of propensity scores!
Examine Within-Study Comparison Studies with different Results The Bulk of the Job Training Comparisons Two Examples from Education
Earliest Job Training Studies: Adding to Smith/Todd Critique Mode of Assignment clearly varied We assume RCT implemented reasonably well But third variable irrelevancies were not controlled, esp location and measurement, given dependence on matching from extant data sets Large initial differences between randomly and non-randomly formed comparison groups Reliance on statistical adjustment to reduce selection, and not initial design
Recent Educational Examples
Agodini & M. Dynarski (2004) Drop-out prevention experiment, 16 m/h schools Individual students, likely dropouts, were randomly assigned within schools—16 replicates Quasi-Experiment—students matched from 2 quite different sources: middle school controls in another study, and national NELS data. Matching on individual and school demographic factors 4 outcomes examined and so in non-experiment 128 propensity scores -16 x 4 x 2--computed basically from demographic background variables
Results Only 29 of 128 cases were balanced matches obtained Why quality matching so rare? In non-experiment, groups hardly overlap. Treatment group is high and middle schools, but comparisons are middle only or from a very non-local national data set Mixed pattern of outcome correspondences in 29 cases of computable propensity scores. Not good OLS did as well as propensity scores
Critique Who would design a quasi-experiment this way? Is a mediocre non-experiment being compared to a good experiment? Alternative design might have been: 1. Regression-discontinuity. 2. Local comparison schools, same selection mechanism to select similar comparison students. 3 Use of multi-year prior achievement data.
Wilde & Hollister (2005) The Experiment—reducing class size in 11 sites; no pretest used at the individual level Quasi-experimental design—individuals in reduced classes matched to individual cases from other 10 sites Propensity scores; mostly demographic Analysis treat each site as a separate experiment And so 11 replicates comparing an experimental and non-experimental effect size
Results Low level of correspondence in experimental and non-experimental effect sizes across the 11 sites So for each site it makes a causal difference whether experiment or quasi-experiment When aggregated across sites, results closer: exp =.68; non-exp = 1.07 But they do reliably differ
Critique Who would design a quasi-exp on this topic without a pretest on same scale as outcome? Who would design it with these controls? Instead select controls from one or more matched schools on prior achievement history Again, a good experiment is being compared to a bad quasi-experiment Who would treat this as 11 separate experiments vs. a more stable pooled experiment? Even the authors, pooled results are much more congruent.
Hypothesis is that... The job training and educational examples that produce different conclusions from the experiment are examples of poor quasi-experimental design To compare good exp to poor quasi-exp is to confound a design type and the quality of its implementation—a logical fallacy But I reach this conclusion ex post facto and knowing the randomized experimental results in advance
Big Conclusions: R-D has given results not much different from experiment in three of three cases. Simpler Quasi-Experiments tend to give same results as experiment if: (a) population matching in the sampling design—Bloom and Aiken studies, or if (b) careful conceptualization and measurement of selection model, as in Shadish et.
What I am not Concluding: That well designed quasi-experiment is as good as an experiment. Difference in: Number and transparency of assumptions Statistical power Knowledge of implementation Social and political acceptance If you have the option, do an experiment because you can rarely put right by statistics what you have messed up by design
What I am suggesting you consider: Whether this be a unit on RCTs or quality causal studies Whether you want to do RDD studies in cases where an experiment is not possible because resources are distributed otherwise Whether you want to do quasi-experiments if group matching on the pretest is possible, as in many school-level interventions?
More Contentiously if: The selection process can be conceptualized, observed and measured very well. An abbreviated ITS analysis is possible, as in Bloom et al. The instinct to avoid quasi-experiments is correct, but it reduces the scope of the causal issues that can be examined
Shadish, Luellen & Clark (2006)
Results-Aiken et al pretest values on SAT/CAT, 2 writing measures Measurement framework the same Pretest ACTs and writing - ns exp vs non OLS tests Results for writing test =.59 and.57 - sig Results for essay =.06 and.16 - ns
Bloom et al Revisited Analysis at the individual level Within city, within welfare to work center, same measurement design Absolute bias- yes Average bias none across 5 within-state sites, even w/o stat tests Average bias limited to small site and non- within-city site-Detroit vs Grand Rapids
Correspondence Criteria Random error and no exact agreement Shared stat sig pattern from zero - 68% Two ESs not statistically different “Comparable” magnitude estimates One as percent of other Indulgence, common sense and mix
Our Research Issues Deconstructing “non-experiment”--do experimental and non-experimental ESs correspond differently for R-D, for ITS, and for simple non-equivalent designs? How far can we generalize results about invalidity of non-experiments beyond job training? Do these within-study comparison studies bear the weight ascribed to them in evaluation policy at DoL and IES?