Stock vs. Flow Sampling in Program Evaluation
Evaluating policy interventions on existing registered clients (stock sample) introduces duration bias because these individuals have already accumulated varying lengths of unemployment and established caseworker habits. Evaluating newly registered clients (flow sample) provides cleaner estimates of initial intervention timing but risks non-random caseworker assignment during field rollout.
Picture this
Imagine testing a new study method in a university. The "stock sample" consists of upperclassmen who have already developed specific study habits and passed or failed several courses. The "flow sample" consists of incoming freshmen starting fresh on day one, allowing researchers to see how the method works from the very beginning.
What the evidence says
Short-term compliance rates remained identical across both subgroups (11% stock vs 14% flow for Definition 1; 29% stock vs 30% flow for Definition 2), confirming that timing of prediction availability did not alter caseworker compliance behavior.
- Who
- N = 22,758 jobseekers in the stock sample (registered before pilot inception) and N = 12,632 jobseekers in the flow sample (registered during pilot rollout) across Swiss employment offices.
- How
- Comparative RCT subgroup analysis evaluating treatment effects and compliance rates separately for stock versus flow jobseekers to control for duration dependence and administrative assignment changes.
What to do
Disaggregate field evaluation samples into distinct stock and flow cohorts to isolate duration dependence and evaluate early-intervention effects independently from long-term unemployment dynamics.
From the source
"We therefore distinguish in the later analyses between a stock and a flow sample. The stock sample contains all jobseekers who were already registered at the beginning of the field study... The flow sample contains all jobseekers who entered later."
Targeting_Labour_Market_Programmes_Results_from_a_Randomized_Experiment.pdf