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EHS 655 Lecture 7: Exposure grouping strategies
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Exposure analysis in a nutshell
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What we’ll cover today Attenuation bias
Sampling issues and temporal trends Exposure groups Stata commands
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ATTENUATION BIAS Heederik, Attfield, 2000
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Repeated measurements
Loomis, Kromhout, 2004
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Ways to reduce attenuation bias
Increase number of exposure measurements per person Increase between-subject variability so inter-individual exposure range is larger
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SAMPLING ISSUES Several different sampling strategies
Haphazard/convenience Worst case Representative Random How exposures were collected may determine what we can infer from them Also need to consider temporal trends Question: what are the strengths and weaknesses of each of these approaches?
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Sampling issues - temporal trends
Heederik, Attfield, 2000
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Sampling issues – temporal trends
Davies, Teschke, Kennedy, Hodgson, Demers, 2008
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Individual-level assessment
Exposure varies greatly over time and space Measure individuals’ exposures Use repeated measurements on individual to calculate individual average (only their data) Often considered gold standard approach Ignacio and Bullock, AIHA, 2006
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Question If individual-level approach is gold standard, come up with at least two reasons we might want to create exposure groups?
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Reasons we might not want individual-level assessments
Even with repeated measurements, measured average level is at best approximation of true exposure N of repeated measurements per individual usually small (scarcity of data) Higher within-person variability and/or smaller inter-person variability = more attenuation of exposure-response relationship Logistical/financial challenges Lack of direct access to individuals
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EXPOSURE GROUPS To address scarce individual data, we commonly pool data across individuals Create group estimates Increased amount of data in groups reduces random variability associated with estimate Apply group estimates to individuals who likely have similar exposures May not be truly applicable to all individuals in group
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Group-level assessment
Create subgroups based on common features of exposure Ideally, all workers within each group measured Must consider between-group, within-group, within-individual variability Tielemans, Kupper, Kromhout, Heederik, Houba, 1998
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Advantages of grouping
Often more effective than individual-level assessment Especially when temporal variability large Logistically less demanding than individual approach Should result in almost unbiased estimated coefficients of exposure-response relationship Attenuation very small with grouped exposure data
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Grouping and attenuation
Seixas and Sheppard, 1996
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Exposure groups Originally “Homogeneous exposure groups” (HEGs)
Current trend is “Similar exposure groups” (SEGs) Groups frequently defined by common structures E.g., job title, work area, activity, behavior, agent, street, etc Individuals in different groups assumed to have different exposures Individuals within groups also have different exposures Differences in activities, behaviors, protective equipment
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Exposure groups Statistical measures of central tendency often applied to groups Mean, median, mode We treat every individual in the group as though they are exposed at the this measure of central tendency For categorical exposures, we assume groups are exposed (1) or unexposed (0) Assumes zero variability in groups
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What do we need to think about when grouping people?
Specificity Summarizes variance within group Large variance = highly specific Precision Summarizes variance between groups Large variance = highly precise
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Exposure grouping goals
Goal 1: create groups which retain true individual differences (specificity) i.e., within-group variability small compared to between-group variability Goal 2: create groups that are as large as possible (precision) Leads to exposure estimates that are more precise than individual worker means
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Tradeoff between specificity and precision
Individual-based strategy Precise (small standard error) but biased estimates of exposure-response relationship Group-based strategy Unbiased (high validity) but imprecise (large standard error) estimates of exposure-response relationship Validity, not precision, most important
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Specificity vs precision (from the reading)
Werner, Attfield, 2000
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Specificity vs precision
Within-group variation Between-group variation Heederik, Attfield, 2000
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Grouping strategies A priori A posteriori
Grouping takes place before measurement is made A posteriori Grouping takes place after measurement is made
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Ways to create exposure groups
Group by Activity type Process Agent Exposure pathway Location Time Etc.
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Example: grouping by agent
Note variation in group size, exposure range, repeated measurements
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Example: grouping by job title
Lewne, Plato, Bellander, Alderling, Gustavsson, 2010
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Example - grouping outcomes
Possibilities Within group variability > between group variability Between group variability > within group variability Question: which of these grouping approaches is preferred? Rappaport, Kromhout, Symanski, 1993
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Group size and attenuation bias
Reducing attenuation bias Increasing number of subjects per group can be as effective as increasing number of measurements per subject
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Question What do you think of this as a grouping strategy, and why?
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Resources EPA ExpoBox
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On to Stata More basic data manipulation commands
Create dummy variables for groups example: tabulate varname, gen(newvarname) Let’s make or identify some groups and compare them
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On to Stata Bivariate analysis examples tabulate varname1 varname2
tab2 varname1 varname2 varname3 tabstat varname, stat(mean sd count) bysort varname1: tabstat varname2, stat(mean sd count) table varname1, contents(mean varnamex sd varnamex) by(varname2) twoway scatter varname1 varname2 Graph matrix varname1 varname2 varname3, half Graph box varname1, over(varname2)
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