Design an experiment to launch fractional shares

Context

You work on a brokerage/investing app. The team is considering launching fractional share trading (users can buy/sell fractions of a stock/ETF instead of whole shares).

Task

1. Explain the expected benefits of fractional shares for the business and for users.
2. Propose an experimentation plan to evaluate the launch:
   • What is the primary success metric ?
   • What diagnostic metrics help explain movement?
   • What guardrail metrics ensure you don’t harm users or the business?
   • Define the unit of randomization (user/account/household/etc.) and the treatment/control experience.
   • Call out key risks: interference/network effects, novelty effects, seasonality, and any selection bias.
3. Sample size / power
   • Show how you would estimate the required sample size for the primary metric (state assumptions such as baseline rate/variance, MDE, $\alpha$ , and power).
   • If you need to adjust the sample size mid-experiment , explain a statistically valid approach.
4. Constraint scenario
   • If the required sample size is too large (you don’t have enough traffic or time), what do you do? Provide at least 3 practical options (design, metrics, variance reduction, ramp strategy, or alternative inference methods) and discuss tradeoffs.

Output

Provide a structured written plan, including metric definitions, assumptions used in power/MDE calculations, and a final recommendation on whether/how to launch.