A city public health department reports the following findings over a five-year period.

A. Ice cream sales and drowning deaths rise and fall together, month over month (r = 0.86).
B. Neighborhoods that received a new protected bike lane saw a 22% drop in cyclist injuries.
C. Patients who received an experimental drug had higher mortality than patients who did not.
D. Schools that adopted a new reading curriculum saw test scores rise 8 points. The district credits the curriculum.
E. Hospitals with the best surgical outcomes are the ones that turn away the most complex cases.
F. The 20 neighborhoods with the highest crime rates last year all saw crime fall this year without any intervention.

For each finding:
1. Name the most plausible causal structure: direct cause, reverse causation, confounding, collider or selection effect, or regression to the mean.
2. Identify the specific lurking variable or mechanism.
3. State the single cheapest study design or additional data point that would distinguish your explanation from the naive one.

Then:
- For finding C, explain confounding by indication in plain language a hospital administrator would understand.
- For finding B, name the one selection effect that would most threaten the conclusion, and say what would rule it out.
- Two of these six share the same underlying structure. Which two, and what is the shared structure?