Drinking Water: Reporting Limits MRLs, MDLs, Significant Figures, and Rounding

1
Drinking Water Reporting Limits (MRLs), MDLs,
Significant Figures, and Rounding

• Presentation to KWWOA
• March 25, 2009
• Department for Environmental ProtectionEnergy
  Environmental Cabinet

To Protect and Enhance Kentuckys Environment
2
Topics to be Covered

• Reporting Limits (MRL)
• Method Detection Limit (MDL)
• Significant Figures
• Rounding Rules

3
Overview

• Laboratories generate the following limits
• IDL
• IDC
• MDL
• Calculated or regulated values
• MRL
• MCL

4
Typical Linear Regression Curve for Direct
Response InstrumentShowing IDL, MDL, MRL, Low
Standard, and MCL
5
Instrument Detection Limit (IDL)

• An instrument detection limit (IDL) is the
  lowest limit that the instrument can detect. It
  is determined on samples which have not gone
  through any sample preparation steps.
• The IDL should always be below the method
  detection limit and can not be used for
  compliance data reporting. The IDL is similar to
  the critical level and criterion of detection
  as defined in Standard Methods 18th Edition.

6
Method Detection Limit

• A statistically derived minimum detection limit
  that was first used by EPA in the early 1970s.
• Several group efforts and trial studies have
  been completed to find another method for
  calculating MDL.

7
Minimum Detection Limit (MDL)

• Requirements in the method must be followed.
  Most methods require initial MDL calculations for
  all analytes and certification officers should
  require the laboratories to calculate their
  detection limits for all regulated contaminants.
  If there is no procedure to determine the
  detection limits in the method, it should be
  determined in accordance with the procedure given
  in 40 CFR 136, Appendix B.

8
MDL Calculation

• Calculated MDL lt Spike Level lt 10x Calculated MDL

9
Method Detection Limits (MDL)
10
MDL Calculation in Excel
11
MDL Calculation in Excel

• SQRT Square Root

Set Range Std Deviation, STDEV S2 Variance, S
SQRT (S2) MDL STDEV Std-t value
STDEV
12
Initial Demonstration of Capability (IDC)

• Requirements in the methods must be followed.
  Before beginning the analysis of compliance
  samples, an initial demonstration of capability
  (IDC) must be performed for each method as
  required in the method.
• An IDC should be performed for each instrument.
  It is also recommended that an IDC be performed
  by each analyst.

13
Reporting Limits (MRL)

• MRL is the lowest concentration of a contaminant
  reportable by a laboratory.

14
Minimum Reporting Limit (MRL)

• The laboratorys minimum reporting limits (MRL)
  should be reported to the client along with the
  data. The reporting limit must be below the MCL.
• Kentucky has simply adopted the Federal Minimum
  Reporting Limit (e.g. Reporting Limit RL).

15
Reporting Limits (MRL)

• Note that the minimum reporting limit (MRL) is
  at, or just above, the lowest calibration
  standard. The MRL must be within the linear
  range of the calibration curve in order to
  maximize its effectiveness over the quality of
  the data reported.

16
Minimum Reporting Limit (MRL)

• Laboratories should run a LFB at their MRL every
  analysis day and should not report contaminants
  at levels less than the level at which they
  routinely analyze their lowest standard.
• While this is a scientifically sound practice,
  whether it is an acceptable practice will depend
  on State and Federal reporting requirements.
• It is important for users of data to understand
  the statistical and qualitative significance of
  the data.

17
Reporting Limits (MRL)

• Complete List Posted on Kentucky Drinking Water
  Website
• Chemistry Analyte MCL / MRL Report Criteria

18
Maximum Contaminant Level (MCL)

• The maximum contaminant level (MCL) is the
  maximum permissible level (concentration) of a
  contaminant in water which is delivered to any
  user of a public water system.
• Refer to 40 CFR Part 141.2 for more details.

19
Significant Figures

• Every measurement has a degree of uncertainty
  associated with it.
• The uncertainty derives from the measuring
  device and from the skill of the person doing the
  measuring.

20
Significant Figure Rules

• Non-zero digits are always significant.
• 2. All zeros between other significant digits are
  significant.

21
Significant Figures Rules (cont)

• 3. The number of significant figures is
  determined starting with the leftmost non-zero
  digit. The leftmost non-zero digit is sometimes
  called the most significant digit or the most
  significant figure. For example, in the number
  0.004205 the '4' is the most significant figure.
  The lefthand '0's are not significant. The zero
  between the '2' and the '5' is significant.

22
Significant Figures Rules (cont)

• 4. The rightmost digit of a decimal number is the
  least significant digit or least significant
  figure. Another way to look at the least
  significant figure is to consider it to be the
  rightmost digit when the number is written in
  scientific notation. Least significant figures
  are still significant! In the number 0.004205
  (which may be written as 4.205 x 10-3), the '5'
  is the least significant figure. In the number
  43.120 (which may be written as 4.3210 x 101),
  the '0' is the least significant figure.

23
Significant Figures Rules (cont)

• If no decimal point is present, the rightmost
  non-zero digit is the least significant figure.
  In the number 5800, the least significant figure
  is '8'.
• Can be written as 5.8 x 103 (2-sig figs)

24
Significant Figure ( and -)

• 32.01 5.325 12 49.335
• However, the sum should be reported as 49.

25
Significant Figures ( and /)

• A density calculation is made in which
• 25.624 grams / 25 mL 1.0 g/mL
• Not as 1.0000 g/mL or 1.000 g/mL.

26
Rounding Round to the Even

• Exactly replicates the common method of rounding
  except when the digit(s) following the rounding
  digit starts with a five and has no non-zero
  digits after it.

27
Rounding Rules

• Decide which is the last digit to keep.
• 2. Increase it by 1 if the next digit is 6 or
  more, or a 5 followed by one or more non-zero
  digits.

28
Rounding Rules (cont)

• Leave it the same if the next digit is 4 or less
• 4. Otherwise, if all that follows the last digit
  is a 5 and possibly trailing zeroes then
  increase the rounded digit if it is currently
  odd else, if it is already even, leave it alone.
  

29
Rounding Examples

• 3.016 rounded to hundredths is 3.02 (because the
  next digit (6) is 6 or more)
• 3.013 rounded to hundredths is 3.01 (because the
  next digit (3) is 4 or less)
• 3.015 rounded to hundredths is 3.02 (because the
  next digit is 5, and the hundredths digit (1) is
  odd)

30
Rounding Examples (cont)

• 3.045 rounded to hundredths is 3.04 (because the
  next digit is 5, and the hundredths digit (4) is
  even)
• 3.04501 rounded to hundredths is 3.05 (because
  the next digit is 5, but it is followed by
  non-zero digits)

31
Putting It All Together

• Significant figures must be used throughout
  calculations.
• Rounding must be performed at the end of the
  calculation.

32
Putting It All Together
33
Questions

• ?

34
Contact Information

• Patrick J. Garrity
• Certification Officer Division of Water200 Fair
  Oaks Lane 4th Floor
• Frankfort, KY 40601
• Phone [phone] ext. 4968
• Fax [phone]
• Email patrick.garrity_at_ky.gov