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Standard Deviation Calculation
Recommended Practice
- Identify control points and critical control points.
- Clean product surfaces.
- Conduct ATP Hygiene Monitoring tests at a certain number of set locations over the course of several days to get 20-50 results. Calculate the average and standard deviation (s.d) for the RLU values.
How to calculate the standard deviation of a single test location based on multiple swab results. Our example will use following RLU readings: {2, 5, 6, 6, 8, 9, 9, 10, 12, 13}.
Step 1. Calculate the mean/ average  :
We have N = 10 because there are ten data points:
X1 = 2
X2 = 5
X3 = 6
X4 = 6
X5 = 8
X6 = 9
X7 = 9
X8 = 10
X9 = 12
X10= 13
 Replacing N with 10
 
 This is the mean.
Step 2. Calculate the standard deviation  :
 Replacing N with 10
 Replacing  with 9
 This is the standard deviation.
- Set limits as:
Pass < (less than) or = (equal to) Mean RLU= 9
Caution > (greater than) or = (equal to) Mean RLU (9) < (less than) Mean RLU + 3 (s.d) (9+3.3= 12.3)
Fail > (greater than) or = (equal to) Mean RLU + 3 (s.d) (9+3.3=12.3)
Pass 0 - 9 RLU
Caution 9 – 12.3 RLU
Fail > 12.3 RLU
- Monitor results and assess trends. Recalculate once a year to optimize limits to improve sanitation/hygiene standards or whenever procedural changes are made to the cleaning program.
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