Interview Questions › Root Cause Analysis

Payment success rate has decreased marginally

Overall payment success rate (PSR) across all merchant checkouts decreased from 86.5% to 84.8% over the past 5 days. Investigate the cause.

Context

The decrease is subtle across aggregate charts, but represents millions in lost gross transaction value across top tier merchants.

Clarifying questions to ask first

• Is the drop uniform across all payment methods or isolated to one rails (e.g., UPI)?
• Which error codes spiked (USER_DROPOUT vs BANK_TIMEOUT vs INSUFFICIENT_FUNDS)?

How to approach it

1. Segment Payment Success Rate by Payment Method (UPI, Credit Cards, Debit Cards, NetBanking, BNPL)
2. Segment by Issuing Bank, Acquiring Gateway, Merchant Category, and Network (Visa, Mastercard, RuPay)
3. Investigate bank downtime, 3D-Secure OTP latency, gateway routing algorithms, and SDK timeout parameters
4. Propose automated failover and dynamic routing improvements

Framework: Payment Method Breakdown -> Bank / Issuer Split -> Gateway Latency & Error Codes -> Dynamic Routing Optimization

Sample answer

• Major private bank core banking maintenance causing UPI timeouts
• New 2FA authentication protocol introducing friction on web checkout
• Gateway smart-router misrouting high-ticket transactions to a degraded acquirer

1. Filter by rails: UPI PSR steady at 91%, Cards dropped from 84% to 76%
2. Filter Cards by Issuer: Bank X 3DS-2 OTP endpoint latency spiked to 22 seconds causing user abandonments

Bank X OTP timeout identified; switched smart routing to fallback acquirer integration and enabled instant WhatsApp OTP fallback.

What interviewers look for

• Payment Success Rate (PSR %)
• Bank Downtime Duration
• OTP Latency
• Gateway Error Codes (Timeout vs Auth Fail)