GIFSQ Tools · Module 10 — Packaging & Shelf-Life · Tool 194 of 500
Fit a straight line to quality scores measured over storage days and read off the rate of change, the fit quality and the day the product hits its limit.
Try sample values — fills the form with the worked example below.
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Change per day
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Fit (R²)
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Days to limit
Equation: Least-squares line: slope = Σ(x − x̄)(y − ȳ) ÷ Σ(x − x̄)²; intercept = ȳ − slope × x̄. R² = 1 − Σ(y − ŷ)² ÷ Σ(y − ȳ)². Days to limit = (limit − intercept) ÷ slope.
Method: Linear regression of a quality attribute against storage time, the usual first analysis of real-time shelf-life data: the line is fitted by least squares and the shelf life is read where the line crosses the quality limit.
Assumptions: quality falls (or rises) in a straight line over the measured range; the limit is a fixed score on the same scale.
Limitations: real quality loss is often curved, not straight — a straight line fitted to early data can overstate shelf life; check the fit (R²) and the residuals before trusting the crossing day.
Rounding: slope and intercept to 4 decimal places; R² to 4 decimal places; days to limit to 2 decimal places; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: Linear regression of a quality attribute against storage time, the usual first analysis of real-time shelf-life data: the line is fitted by least squares and the shelf life is read where the line crosses the quality limit.
Assumptions: quality falls (or rises) in a straight line over the measured range; the limit is a fixed score on the same scale.
Limitations: real quality loss is often curved, not straight — a straight line fitted to early data can overstate shelf life; check the fit (R²) and the residuals before trusting the crossing day.
Rounding: slope and intercept to 4 decimal places; R² to 4 decimal places; days to limit to 2 decimal places; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: Scores 100, 90, 80, 70 at days 0, 30, 60, 90: slope = −0.3333 per day, intercept 100, R² = 1.0000; with a limit of 70 the line crosses at day 90.00.
Enter values above to see results.
A high R² on three points proves little — use at least four time points and check the residuals, not just R². Tool 194 of 500 — GIFSQ
