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Data handling. Sabine Beulke, Central Science Laboratory, York, UK Kinetic Evaluation according to Recommendations by the FOCUS Work Group on Degradation Kinetics Washington, January 2006. Outline. Data quality Replicates Concentrations below LOD or LOQ Experimental artefacts Outliers

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Data handling

Data handling

Sabine Beulke, Central Science Laboratory, York, UK

Kinetic Evaluation according to Recommendations by the FOCUS Work Group on Degradation Kinetics

Washington, January 2006


Outline
Outline

  • Data quality

  • Replicates

  • Concentrations below LOD or LOQ

  • Experimental artefacts

  • Outliers

  • Time zero samples

  • Data weighting

    For more information see

    Chapter 6.1 of the FOCUS report


Data quality
Data quality

  • Dissipation pattern and - for metabolites and sediment data - the increase, plateau and decline phase must be clearly established

  • No. of data points (n) >> no. of parameters (p)

    (theoretical minimum n = p+1, but this is often not sufficient)

  • The better the quality the smaller the no. of datapoints needed


Replicates
Replicates

  • Use true replicates individually in the optimization

  • Average replicate analytical results from same sample prior to curve fitting

  • Average all replicates prior to calculating 2 statistics


Concentrations below lod or loq
Concentrations below LOD or LOQ

Parent in soil, total water-sediment system, water column

  • Set all concentrations between LOD and LOQ

    to measured value or 0.5 x (LOD+LOQ)

  • Set first sample < LOD to 0.5 x LOD

  • Omit samples after first non-detect unless later samples > LOQ

Set to measured value

Set to 0.5 x LOD

Omit


Concentrations below lod or loq1
Concentrations below LOD or LOQ

Metabolite in soil and parent and metabolite in sediment

  • Set time zero samples < LOD to 0

  • Set sample <LOD just before & after detectable amount to 0.5 LOD

  • Omit all other samples < LOD (exceptions)

  • Set concentrations between LOD and LOQ to measured value or 0.5 x (LOD+LOQ)


Experimental artefacts
Experimental artefacts

  • Discard results clearly arising from analytical or procedural errors before analysis

  • If microbial activity declined significantly during study:

    Include all data initially, then exclude later sampling points and repeat fitting


Outliers

DT50 42 days

DT50 34 days

Outliers

  • Include all data in curve fitting as a first step

  • Omit outliers based on expert judgement

  • Statistical outlier test where possible


Time zero samples
Time zero samples

  • Include initial amount of parent (soil, total w/s system and water column) in parameter estimation as a first step

M0 variable: DT50 = 68 days

Note: This hypothetical dataset is not described well by SFO kinetics and is only used to illustrate the effect of fixing or estimating the initial concentration.

M0 fixed: DT50 = 48 days


Time zero samples1
Time zero samples

  • Add time-zero concentrations of metabolites > 0 to parent unless due to impurity in application solution

  • Add time-zero concentrations > 0 of parent or metabolite in sediment to water


Data weighting
Data weighting

No transformation: DT50 = 51 days

No transformation: DT50 = 54 days

Log transformed: DT50 = 57 days

Log transformed: DT50 = 108 days

Always use unweighted data as a first step!

Note: This hypothetical dataset is not described well by SFO kinetics and is only used to illustrate the effect of log-transformation.


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