Data handlingData handling

Sabine Beulke, CSL, York, UK

FOCUS Work Group on Degradation Kinetics

Estimating Persistence and Degradation Kineticsfrom Environmental Fate Studies in EU Registration

Brussels, 26-27 January 2005

OutlineOutline

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 qualityData 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

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Days after application

% o

f app

lied

radi

oac

tivity

Parent Metabolite

ReplicatesReplicates

Use true replicates individually in the optimisation

Average replicate analytical results from same sample prior to curve fitting

Average all replicates prior to calculating 2 statistics

Concentrations below LOD or LOQConcentrations 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

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Set to measured value

Set to 0.5 x LOD

Omit

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Concentrations below LOD or LOQConcentrations below LOD or LOQMetabolite in soil and parent and metabolite in sediment

Set time zero samples < LOD to 0

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

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

Omit all other samples < LOD (exceptions)

Experimental artefactsExperimental 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

Concentration vs. time plot

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0 20 40 60 80 100Time

Co

nce

ntr

atio

n

measured

SFO

Outlier included

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Co

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ntr

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Outlier excluded

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ntr

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nmeasured

SFO

DT50 42 days DT50 34 days

OutliersOutliers

Include all data in curve fitting as a first step

Omit outliers based on expert judgement

Statistical outlier test where possible

Time zero samplesTime zero samples Include initial amount of parent (soil, total w/s system and

water column) in parameter estimation as a first step

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Days after application

Soi

l res

idue

(m

g/kg

) M0 variable: DT50 = 68 days

M0 fixed: DT50 = 48 daysNote: 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.

Time zero samplesTime 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

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Days after application

Soi

l res

idue

(m

g/kg

)

M0 variable: DT50 = 68 days

M0 fixed: DT50 = 48 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.

Data weightingData weightingAlways use unweighted data as a first step!

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Days after applicationS

oil r

esid

ue (

mg/

kg)

Measured No transformation Log transformed

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Days after application

Soi

l res

idue

(m

g/kg

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Measured No transformation Log transformed

No transformation: DT50 = 51 days

Log transformed: DT50 = 57 days

No transformation: DT50 = 54 days

Log transformed: DT50 = 108 days

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