How do I tell whether a comparison uses the same time period for both groups?

Start with the dates, not just the labels. For each group, find the exact start and end dates, then check whether the intervals have the same duration and use the same definition of a year or other time unit. Also look for notes about missing records or partial coverage. A label such as “2023” may not tell you enough on its own.

A useful first check is to write the windows as explicit ranges, such as “January 1–December 31, 2023.” Then compare those ranges. If one figure gives only a year label and the other gives exact dates, mark the first window as unclear rather than assuming they match.

Check whether both groups use the same kind of period and the same calculation method. For example, are values grouped by calendar year, school year, season, or another defined cycle? Are both figures totals, averages, or counts over that period? Then check whether the underlying records actually cover the full stated interval.

For a specific example, USGS Water Services documentation says that a statistic can depend on specified start and end dates; without them, that service may use its period of record. It also distinguishes calendar years from water years, which run October 1 through September 30, and notes that calculations use data actually available. These are checks grounded in that USGS service’s documentation, not a universal rule for every dataset. [S1] You still need to inspect each figure’s own definitions and coverage.

Here is an invented example. Assume these are counts from a fictional club:

• Group A is labeled “2023,” covers January–December 2023, and has a count of 48.
• Group B is also labeled “2023,” covers October 2023–September 2024, and has a count of 53.

The labels look alike, but the stated date ranges do not match: Group A covers a calendar year, while Group B covers a different 12-month window. The counts therefore are not a clean same-period comparison. The example also does not say whether every day in either range has records or exactly how the counts were gathered.

A helpful repair is to request both groups for the same explicit dates and the same counting rule. If that is not possible, show the actual ranges beside the figures and describe the comparison as limited, rather than presenting the values as if they covered identical windows.

Don’t fill in the gaps by guessing. Ask for the start and end dates, the definition of the period, the calculation used, and whether records cover the full interval. If you cannot get those details, label the time window as unknown and avoid drawing a firm conclusion from the side-by-side figures.

You can copy this request: “Please confirm the exact start and end dates for each figure, how the period is defined, whether both figures use the same calculation, and whether records cover the full stated interval. If possible, please recalculate both using [shared start date] through [shared end date].”