A useful international price comparison needs the same product, the same unit, a known observation date and a consistent currency conversion. Without those four checks, a neat ranking can compare different purchases.
1. Define the purchase
Record the model, pack size, billing interval and conditions. A 330 ml can is not a 500 ml bottle. A monthly plan is not an annual promotional offer. A phone with a trade-in requirement is not an unconditional retail price.
2. Preserve the original amount
Keep the local price and currency alongside the converted value. Preserve the source label, source URL if available, observation date and any tax information. An import timestamp is useful for system maintenance but is not the date a retailer charged that price.
3. Calculate the relative difference correctly
Hypothetical example: country A costs $12 and country B costs $10. A is 20% more expensive than B: (12 − 10) / 10 × 100. B is 16.7% cheaper than A: (10 − 12) / 12 × 100. The percentage changes when the reference country changes. The dollar difference remains $2.
4. Keep missing values missing
If a comparable item is unavailable, record it as missing. Replacing a missing price with zero falsely makes that country the cheapest. Do not silently switch to a different product to fill a table.
5. Publish enough evidence to reproduce the answer
A reader should be able to recover the unit, currency, scope and source status without guessing. Include a clear explanation of any outlier handling or weighting. Keep a previous dataset version when publishing a new one so changes can be checked.
Our reference dataset explicitly labels unknown observation dates. It is a starting point for transparent comparison, not a substitute for new evidence.