3 sampling elo n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
3. Sampling (ELO) PowerPoint Presentation
Download Presentation
3. Sampling (ELO)

Loading in 2 Seconds...

play fullscreen
1 / 11

3. Sampling (ELO) - PowerPoint PPT Presentation


  • 49 Views
  • Uploaded on

3. Sampling (ELO).

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about '3. Sampling (ELO)' - zasha


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
3 sampling elo
3. Sampling (ELO)

“A defined procedure whereby a part of a substance, material or product is taken to provide for testing or calibration a representative sample of the whole. Sampling may also be required by the appropriate specification for which the substance, material or product is to be tested or calibrated.”

ie The sample(s) must be REFLECTIVE of the TRUE situation

Background Reading: Quality Assurance in Analytical Chemistry, Chapter 3

For an example relevant to your analyses, see BS EN ISO 707:2008 Milk and milk products – Guidance on sampling

sampling strategies 1
Sampling Strategies (1)

Depend on:

  • Types of parent material
    • Homogeneous or heterogeneous
    • Static (stable and contained) or dynamic (cannot be resampled)
  • Concentration
    • Trace (prone to heterogeneity and contamination)
    • Principal component (% uncertainty much lower)
  • Packaging
    • Bulk (eg a silo)
    • Packaged (eg cornflakes)
    • Items (eg tablets)
  • Results required
    • Quantitative (how much)
    • Qualitative or compliance (yes/no answer) – “acceptance sampling”
sampling strategies 2
Sampling Strategies (2)
  • Criteria?
    • attributes (x% of items must conform)
    • variables (a specified average and sd)
  • Selective sampling OK? (eg fruit but not stone)?

Overall strategy determined by purpose:

  • Client requirements
  • Statutory requirements (legal obligations)
  • Trade definitions (contractual)
types of sampling
Types of Sampling
  • 100% sampling (all items monitored)
  • Probability sampling (statistical chance of being representative)
    • Judgement (guided by the reason for analysis)
    • Quota (stratified judgement sampling)
    • Convenience (as and when available)
  • Non-probability sampling (selective)
sample size and number
Sample size and number

Must be specified as part of the method.

  • Sample Size
    • Enough for method
    • Enough to be reflective (esp for trace analysis)
  • Sample number
    • enough to reduce uncertainty (esp that from sampling) to acceptable levels
sampling uncertainty
Sampling Uncertainty

Overall uncertainty in analysis is composed of:

  • Measurement uncertainty (quantifiable using standards)
  • Sample uncertainty which is caused by
    • Population uncertainty (real wobble in the system)
    • Sampling uncertainty (due to the process of collection)

Sampling uncertainty must be reduced until:

  • it does not obscure population uncertainty
  • the resource required outweighs the risk of an incorrect result

More samples and smaller particles reduce sampling uncertainty

See also CH250:1 Causes of uncertainty

how many samples are needed
How many samples are needed?

Number of samples needed can be estimated by rearranging the equation for confidence intervals:

  • n is the number of samples
  • t is Student’s t (routinely approximated to 2 for 95% confidence)
  • s is the standard deviation for the method (measured using reference standards, and inter-laboratory trials)
  • E is the size of the effect that must be measurable (in the same units as s).
number of samples worked eg
Number of samples - worked eg

Cadmium concentrations in soil were measured at a brownfield site during preliminary surveying. The mean ± standard deviation were (16 ± 4) ppm.

a) Calculate the number of samples required to obtain a total uncertainty of 20%.

b) How many samples would be required to reduce this uncertainty to 10%?

number of samples answer
Number of samples - Answer

Cadmium concentrations in soil were measured at a brownfield site during preliminary surveying. The mean ± standard deviation were (16 ± 4) ppm.

a) Calculate the number of samples required to obtain a total uncertainty of 20%.

Between 6 and 7, assuming t=2

b) How many samples would be required to reduce this uncertainty to 10%?

25, assuming t=2

how big should the sample be
How big should the sample be?

There are various statistical calculations based on a) the proportion of analyte present and b) the particle size to approximate sample mass needed. The simplest of these is as follows:

Where Ks is the sampling constant, m is the mass in g and %CV is the Coefficient of Variation (which must be calculated from measurements on several test portions).

how big should the sample be1
How big should the sample be?

Once Ks is known the equation can be rearranged to calculate:

a) The test portion mass (m) required to achieve a specified %CV

b) The likely %CV from a given test portion mass