All observations related to the water system and environmental monitoring are subject to trend analysis. Trends are prepared but its limit always remains a point of discussion because all regulatory guidelines provide a single limit for the tests. No alert and action limit is given.
There are many formulas to determine the in-house alert and action limits but 3 sigma formula is more reliable and widely used formula.
To determine these limits first of all standard deviation of the result values is calculated and then it is added in the average of all values. For example:
If you have 10 user points of purified water having the official limit 100 cfu/ml and has a data of 15 days, calculate the alert and action limits in following way.
Average of all results = 11.28
Standard deviation (Sigma) = 4.16
Alert Limit = Average+(2xSigma)
=11.28+(2x4.16)
=20 cfu/ml
Action Limit = Average+(3xSigma)
=11.28+(3x4.16)
=24 cfu/ml
Sampling Point
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Results of Different Days
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1st
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2nd
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3rd
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4th
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5th
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6th
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7th
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8th
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9th
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10th
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11th
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12th
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13th
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14th
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15th
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SP01
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05
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08
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11
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06
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08
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05
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12
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06
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06
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10
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12
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11
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06
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13
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17
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SP02
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08
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13
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06
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11
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08
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09
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07
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10
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08
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06
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13
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11
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07
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09
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06
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SP03
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12
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19
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08
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12
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13
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09
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07
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15
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12
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10
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10
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13
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09
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07
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12
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SP04
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18
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10
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09
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13
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06
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09
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11
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10
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12
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08
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12
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08
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06
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11
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09
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SP05
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12
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08
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11
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08
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04
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15
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09
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06
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12
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10
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07
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13
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16
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10
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12
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SP06
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05
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12
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08
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06
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06
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19
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12
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10
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09
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11
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08
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08
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08
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12
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16
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SP07
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10
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13
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09
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16
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12
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08
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11
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10
|
14
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24
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18
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21
|
16
|
12
|
16
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SP08
|
16
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18
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08
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11
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22
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12
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09
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14
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12
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06
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10
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16
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20
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11
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13
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SP09
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09
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18
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22
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09
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14
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06
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13
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09
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14
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12
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07
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13
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16
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09
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11
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SP10
|
15
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22
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08
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12
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09
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13
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17
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20
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14
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08
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08
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13
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11
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19
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22
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Results should not exceed the alert limit and investigation should be done whenever it exceeds. Result exceeding the action limit shows the serious problem in the water system and must be rectified immediately.
Note: Sufficient data should be available to determine the alert and action limits by 3 sigma formula. Alert and action limit of water system and environmental monitoring should be calculated with minimum one year data because results vary in different seasons of the year.
WHO (TRS-970 page-85) also suggests establishing the alert and action limits for water system and trending should not exceed 2 sigma. An investigation for the results exceeding the alert limit should be done for the root cause. Appropriate corrective action should be taken to prevent the cause.
Download 3 sigma formula in excel calculation sheet.
As per TRS-970 established action and alert limit for water system should be exceed 2 sigma. what is the meaning of 2 sigma and 3 sigma calculation.
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