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Applying impact evaluation tools

Applying impact evaluation tools. A hypothetical fertilizer project. 5 second review. To do an impact evaluation, we need a treatment group and a comparison group

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Applying impact evaluation tools

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  1. Applying impact evaluation tools A hypothetical fertilizer project

  2. 5 second review • To do an impact evaluation, we need a treatment group and a comparison group  We need a comparison group that is as identical in observable and unobservable dimensions as possible, to those receiving the program, and a comparison group that will not receive spillover benefits.

  3. Example: providing fertilizer to farmers • The intervention: provide fertilizer to farmers in a poor region of a country (call it region A) • Program targets poor areas • Farmers have to enroll at the local extension office to receive the fertilizer • Starts in 2002, ends in 2004, we have data on yields for farmers in the poor region and another region (region B) for both years

  4. How to construct a comparison group – building the counterfactual • Randomization • Matching • Difference-in-Difference • Instrumental variables • Regression discontinuity

  5. 1. Randomization • Individuals/communities/firms are randomly assigned into participation • Counterfactual: randomized-out group • Advantages: • Often called the “gold standard”: by design: selection bias is zero on average and mean impact is revealed • Perceived as a fair process of allocation with limited resources • Disadvantages: • Ethical issues, political constraints • Internal validity (exogeneity): people might not comply with the assignment (selective non-compliance) • Unable to estimate entry effect • External validity (generalizability): usually run controlled experiment on a pilot, small scale. Difficult to extrapolate the results to a larger population.

  6. Randomization in our example… • Simple answer: randomize farmers within a community to receive fertilizer... • Potential problems? • Run-off (contamination) so control for this • Take-up (what question are we answering) • Generalizability – how comparable are these farmers to the rest of the area we would consider providing this project to • Randomization wasn’t done right • Give farmers more fertilizer, they plant more land (and don’t use the right application) – monitor well…

  7. 2. Matching • Match participants with non-participants from a larger survey • Counterfactual: matched comparison group • Each program participant is paired with one or more non-participant that are similar based on observable characteristics • Assumes that, conditional on the set of observables, there is no selection bias based on unobserved heterogeneity • When the set of variables to match is large, often match on a summary statistics: the probability of participation as a function of the observables (the propensity score)

  8. 2. Matching • Advantages: • Does not require randomization, nor baseline (pre-intervention data) • Disadvantages: • Strong identification assumptions • Requires very good quality data: need to control for all factors that influence program placement • Requires significantly large sample size to generate comparison group (and same survey to comparison and treatment is important)

  9. Matching in our example… • Using statistical techniques, we match a group of non-participants with participants using variables like gender, household size, education, experience, land size (rainfall to control for drought), irrigation (as many observable characteristics not affected by fertilizer)

  10. Matching in our example…2 scenarios • Scenario 1: We show up afterwards, we can only match (within region) those who got fertilizer with those who did not. Problem? • Problem: select on expected gains and/or ability (unobservable) • Scenario 2: The program is allocated based on historical crop choice and land size. We show up afterwards and match those eligible in region A with those in region B. Problem? • Problems: same issues of individual unobservables, but lessened because we compare eligible to potential eligible • now unobservables across regions

  11. 3. Difference-in-difference • Observations over time: compare observed changes in the outcomes for a sample of participants and non-participants • Identification assumption: the selection bias is time-invariant (‘parallel trends’ in the absence of the program) • Counter-factual: changes over time for the non-participants Constraint: Requires at least two cross-sections of data, pre-program and post-program on participants and non-participants • Need to think about the evaluation ex-ante, before the program • Can be in principle combined with matching to adjust for pre-treatment differences that affect the growth rate

  12. Implementing differences in differences in our example… • Some arbitrary comparison group • Matched diff in diff • Randomized diff in diff • What would we want to look out for? • These are in order of more problems  less problems, think about this as we look at this graphically

  13. As long as the bias is additive and time-invariant, diff-in-diff will work ….

  14. What if the observed changes over time are affected?

  15. 4. Instrumental Variables • Identify variables that affects participation in the program, but not outcomes conditional on participation (exclusion restriction) • Counterfactual: The causal effect is identified out of the exogenous variation of the instrument • Advantages: • Does not require the exogeneity assumption of matching • Disadvantages: • The estimated effect is local: IV identifies the effect of the program only for the sub-population of those induced to take-up the program by the instrument • Therefore different instruments identify different parameters. End up with different magnitudes of the estimated effects • Validity of the instrument can be questioned, cannot be tested.

  16. IV in our example • It turns out that outreach was done randomly…so the time/intake of farmers into the program is essentially random. • We can use this as an instrument • Problems? • Is it really random? (roads, etc)

  17. 5.Regression discontinuity design • Exploit the rule generating assignment into a program given to individuals only above a given threshold – Assume that discontinuity in participation but not in counterfactual outcomes • Counterfactual: individuals just below the cut-off who did not participate • Advantages: • Identification built in the program design • Delivers marginal gains from the program around the eligibility cut-off point. Important for program expansion • Disadvantages: • Threshold has to be applied in practice, and individuals should not be able manipulate the score used in the program to become eligible.

  18. RDD in our example… • Back to the eligibility criteria: land size and crop history • We use those right below the cut-off and compare them with those right above… • Problems: • How well enforced was the rule? • Can the rule be manipulated? • Local effect

  19. To sum up • Use the best method you can – this will be influenced by local context, political considerations, budget and program design • Watch for unobservables, but don’t forget observables • Keep an eye on implementation, monitor well and be ready to adapt

  20. Thank you

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