Statistical Methods
Document the two observations supporting an interpolated value
Define the task
Keep an observation flag beside each reported value. An estimated measurement between scheduled checks can look identical to an observed one once a spreadsheet rounds it. Begin with one series in consistent units and list its observed coordinates in ascending order. Resolve repeated times using the acquisition protocol before fitting a line.
Carry out the check
For a requested time, identify the nearest earlier and later observed times in the same series. Divide elapsed time since the earlier reading by the total bracket duration. Add that fraction of the change to the earlier value. Store the two supporting coordinates, the gap length and any maximum allowed gap. Leave an out-of-range request unresolved unless a separate extrapolation rule has been justified.
Worked hypothetical example
In a hypothetical calibration run, readings are 10 units at minute 2 and 22 units at minute 8. Minute 4 lies one third of the way across the bracket, giving 14 units. A documented recalibration at minute 3 would invalidate this interpolation. A straight line assumes continuity across the bracket and supplies no uncertainty interval.
Method checked 10 November 2026 against R documentation: Interpolation Functions.