The Measurement Process

1
The Measurement Process

• How Do You Get Started?

2
Introduction

• Measurements must be
• Reliable
• Defensible
• Of known quality.
• Sampling and Analysis Plan (SAP)
• It is crucial that these address the objectives
  of the study.

3
Introduction

• The factors that must be considered when
  assessing reliability and relevance include
• Whether the method used is accepted by the
  scientific community
• Whether the method has been peer reviewed
• Whether the method can be or has been tested
• Whether there are standards controlling the
  methods performance
• Whether the known or potential rate of error is
  acceptable.

4
Measurement Process

• The environmental measurement process consists of
  three phases
• Planning
• Implementation
• Assessment.

5
Measurement Process

• Measurement of an environmental process requires
  many levels of detail be properly addressed.
• See Handout.
• All of these may not apply to every forensic
  study, but should be considered prior to
  exclusion.

6
Measurement Process

• Understanding the measurement process and the
  errors associated with it are key to the
  collection of accurate measurements.
• Categories of errors
• BLUNDERS
• Systematic
• Random

7
Planning Errors

• There are eight areas of planning, implementation
  and data handling activities where errors occur
  in the measurement process.

8
Errors in Planning

• Errors in determining the correct question.
• Resolve this at the beginning of the measurement
  process
• The chosen process may answer only one question
  at a time.
• Take care when using data from one question to
  answer anothererror.

9
Errors in Planning

• 2. Errors in selecting the decision unit.
• A decision unit is the media or portion of the
  media that you are trying to address in the
  investigation.
• For identification, you may select the portion of
  the media likely to give a clear chemical
  signature.
• For , the decision unit is the entire volume
  of material where the analyte is of interest
  (pile, lagoon, drum).

10
Errors in Planning

• Errors in selecting the correct confidence.
• Aspects to confidence Type of confidence and
  level of confidence.
• Two types include Statistical confidence and
  professional confidence.

11
Errors in Implementation

• 4. Errors in sampling the decision unit.
• Drum or site? Must have heterogeneity.
• Two types of heterogeneity
• Constitutional heterogeneity
• Differences in analyte
• Distributional heterogeneity
• Non-random distribution in a sample
• Controlled through multi-incremental sampling.

12
Errors in Implementation

• Errors in preserving the integrity of the sample
  prior to analysis.
• Errors in collecting the analytical sub-sample
  from the field.
• Errors in sample analysis
• Precision and bias, method error, lab error, data
  management error.
• Error in data handling
• QA/QC
• Data validation.

13
Planning

• Use data quality objectives (DQOs).
• What you want to achieve
• What you will leave behind (PRGs)
• DQOs should include
• Analytical uncertainty
• Uncertainty in health-based standards
• Exposure pathways
• Sample collection.

14
DQO Formal Procedures (EPA, ASTM)

• Step 1 State the problem.
• Step 2 Id the decision.
• Step 3 Id the inputs to the decision.
• Step 4 Define the boundaries of the study.
• Step 5 Develop tolerable limits on decision
  error.
• Step 6 Specify tolerable limits on decision
  error.
• Step 7 Optimize the design for obtaining data.

15
Planning

• The three most important inputs to developing a
  properly designed study
• Determining the question
• Population
• Confidence.
• SEE TABLE 4.3.

16
Questions

• The purpose of forensic sampling should be
  clearly defined prior to sample selection.
• One question The id of specific contaminants or
  ratios of contaminants can be used to determine
  its source or the age of the released materials.

17
Questions

• Another question
• The of a contaminant within a decision unit
  may be used to determine the total amount of
  material used/discharged or to pinpoint a source
  by discerning gradients.
• Determining the ID or of contaminants is
  crucial to determining the who, what, where and
  when questions.

18
Decision Units

• After determining the forensic question, a
  decision unit should be identified.
• The decision unit is the total material tha the
  analytical results apply to.
• Can be lt 1 g to gt 1000s of kg
• This is a difficult and important step,
• Involves judgment and knowledge--- it could be a
  stained area, a phase of a liquid, an individual
  particle, an entire lagoon or waste pile.

19
Confidence

• After the question and decision unit are
  selected, the desired type and level of
  confidence should be specified.
• Statistical confidence
• Professional judgment.
• Confidence in a decision is a function of all the
  errors in the entire measurement process.

20
Sampling Plan Development

• After completing the DQO process, a
  scientifically-based, defensible SAP using field
  sampling theory is important.
• Sampling theory is critical in the determination
  of sampling error.
• Without controlling sampling error, the data is
  in question.
• Increased variability
• Underestimation of the mean
• Higher rate of QC failure.

21
Sampling for Identification

• There are five potential areas where sampling
  error can occur.
• Selecting the decision unit.
• Selecting the confidence.
• Collecting the sample form the decision unit.
• Preserving the integrity of the sample prior to
  analysis.
• Collecting the analytical sub-sample from the
  field sample.

22
Quality Control Measures

• Trip Blanks
• Field Blanks
• Decontamination Blanks
• Split samples
• Replicates

23
Implementation

• Sampling
• Soil gas
• Soil
• Sediment
• Groundwater
• Surface Water
• Surfaces
• Ambient Air

24
Sample Handling

• Break out the laptops.
• Look up SW-846
• Examine the sample handling text and guidance.
• Examine Tables 4.7, 4.8, 4.9, 4.10.