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Affirm·Data Scientist·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

Affirm data scientist interview with a pandas-based credit risk question. Pretty applied, felt more like a take-home vibe even though it was probably a live screen. The problem was grounded in real fintech logic which I appreciated.

Questions Asked (1)

Q1

Given a loan payments dataset, use pandas to compute: (a) total amount paid per loan, (b) payment success rate per loan, and (c) a boolean flag showing whether the loan's full $1,000 principal has been repaid. Return the resulting DataFrame.

Product Analytics & MetricsData Modeling
Author's notes

The groupby part was fine, agg with a dict of functions is second nature at this point.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the dataset schema and definitions (e.g., what constitutes a successful payment, how to handle refunds or partial payments). Then use pandas groupby aggregations to compute total amount paid and success rate per loan, and finally derive the boolean flag by comparing total paid to the $1,000 principal. Ensure the output is a clean DataFrame with one row per loan.

Pro tip: Always validate assumptions about the data, such as whether 'amount paid' includes only successful payments or all attempts, and whether the principal is fixed at $1,000 for all loans. Mentioning these checks shows attention to detail and business acumen.

1. Understand the data and requirements

Inspect the dataset to identify columns like loan_id, payment_amount, payment_status, and any other relevant fields. Clarify definitions: what counts as a successful payment? Is the principal always $1,000? Are there refunds or adjustments?

2. Compute total amount paid per loan

Filter to successful payments if necessary, then group by loan_id and sum the payment amounts. Use pandas groupby and agg to get total_paid per loan.

3. Calculate payment success rate per loan

For each loan, compute the proportion of successful payments out of total payment attempts. This can be done by creating a boolean column for success, then grouping by loan_id and taking the mean.

4. Create boolean flag for full principal repayment

Compare the total amount paid per loan to the $1,000 principal. Create a new column 'fully_repaid' that is True if total_paid >= 1000, else False.

5. Assemble and return the final DataFrame

Merge the aggregated metrics into a single DataFrame with one row per loan, containing loan_id, total_paid, success_rate, and fully_repaid. Ensure the output is clean and ready for analysis.

Key Points to Mention

  • Handling of payment statuses: define what 'success' means (e.g., status == 'completed') and filter accordingly.
  • Edge cases: loans with no payments, partial payments, overpayments, or refunds.
  • Use of groupby and aggregation functions (sum, mean, count) for efficiency.
  • Data types: ensure payment amounts are numeric and success flags are boolean.
  • Business context: why success rate and full repayment are important metrics for Affirm (e.g., risk assessment, customer behavior).
  • Output format: a DataFrame indexed by loan_id or with loan_id as a column, with clear column names.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.