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The road map to problem solving The analysis plan FETP India

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Competency to be gained from this lecture Plan the analysis of a study on the basis of the study objectives

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Key areas Objectives of the study Design and indicators Study parameters Analysis Sample size

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The ad-hoc approach to conducting an epidemiological study Before data collection I want to do a study I am not clear about the objectives I prepare a questionnaire I am not clear about what information I need I collect data I am not clear what I will use for what After data collection I come back with data I realize they are difficult to analyse I analyse the data I realize it is difficult to interpret the results I interpret the results I realize it is difficult to use them Sound familiar?

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The life cycle of an epidemiological investigation Identifying data needs Spelling out the research question Formulating the study objectives Planning the analysis Preparing data collection instruments Analysing data Drawing conclusions Formulating recommendations Involving the programme Collecting data Analysis plan

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The analysis plan: A road map to making sense of data 1.Formulate the objectives of the study 2.Choose a design to identify key indicators 3.Identify parameters needed for indicators 4.Prepare the analysis 5.Estimate sample size Objectives

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The analysis plan: A road map to making sense of data 1.Formulate the objectives of the study 2.Choose a design to identify key indicators 3.Identify parameters needed for indicators 4.Prepare the analysis 5.Estimate sample size Objectives

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The study objectives Formulated in limited number Sorted out as primary and secondary Focused No more than one verb each Clear about whether: Hypothesis testing Quantity measuring Epidemiological terms Objectives

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Estimating versus testing Estimating a quantity Use the verb “Estimate” E.g., Estimate the prevalence of diabetes Testing a hypothesis Use the verb “Determine” E.g., Determine whether a contaminated well caused an outbreak Objectives

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Good and bad examples of study objectives Determine the importance of Kala Azar Estimate the prevalence of Kala Azar in the community Assess vitamin A deficiency and tuberculosis Estimate the effect of vitamin A supplementation over the cure rate of tuberculosis patients Evaluate iodine deficiency and equity Determine whether iodine deficiency is more common among poorer people Objectives

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From testing a hypothesis to estimating a quantity Determine whether iodine deficiency is more common among poorer people Hypothesis testing Crude objective, smaller sample size Estimate the relative frequency of iodine deficiency among poorer people Quantity estimating More elaborate objective, larger sample size Objectives

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The analysis plan: A road map to making sense of data 1.Formulate the objectives of the study 2.Choose a design to identify key indicators 3.Identify parameters needed for indicators 4.Prepare the analysis 5.Estimate sample size Design and indicators

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Elements to consider to choose a study design Is the study descriptive or analytical? Is there a need to compare groups? Is there just a need to estimate a frequency? Is the outcome (e.g., disease) acute or chronic Need of prevalence data for chronic outcomes Need of incidence data for acute outcomes Is the outcome common or rare? Case control design for rare outcomes Cohort / cross sectional designs for common outcomes Design and indicators

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Choosing a study design adapted to the objective to identify the indicator

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Example: Estimating the relative frequency of iodine deficiency among people below poverty line (BPL) Elements deducted from the objective: Analytical approach: Compare two groups Chronic condition: Prevalence data Common condition: Survey Study design: Analytical cross sectional study Indicator: Ratio of prevalence of iodine deficiency among BPL persons Design and indicators

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The analysis plan: A road map to making sense of data 1.Formulate the objectives of the study 2.Choose a design to identify key indicators 3.Identify information needed for indicators 4.Prepare the analysis 5.Estimate sample size Parameters

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Identification of information needed to calculate the indicator List the indicators that the study will generate Rates, ratio, proportions or quantitative variables Example: Measles coverage Identify the information elements that will be needed to calculate the indicators Numerators an denominators Example: Number of children vaccinated / total children Information elements may address: Outcome variable (s) “Covariate”, including Potential risk factors Potential confounders Parameters

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From information element to variables Identify the variables that may be used to reflect the information element The information element “Measles vaccination status” can be assessed by review of cards or interview of the mother Choose the best possible variable Review standardized guidelines (e.g., WHO, CDC) Plan data collection methods for each variable Observation Interview Laboratory methods Parameters

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Example: Outcome measurement for iodine deficiency study Information element Variable Data collection method to obtain the variable Past exposure to iodine deficiency Goitre Physical examination Current iodine intake Urine iodine excretion Laboratory Access to iodized salt Test of house salt for iodization Field spot test Parameters

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Example: Covariate measurement for iodine deficiency Potential risk factors Income (Validated field methods) Community (e.g., minorities) Caste Education Residence Potential confounding factors Age Sex Parameters

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The analysis plan: A road map to making sense of data 1.Formulate the objectives of the study 2.Choose a design to identify key indicators 3.Identify parameters needed for indicators 4.Prepare the analysis 5.Estimate sample size Analysis

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Rationale for preparing the data analysis in advance Focus on the objectives of the study Limit multiple comparisons Avoid comparisons for which the study was not designed Ensure that data collected can be analyzed “Other, specify: _____” kind of data that create minuscule groups that cannot be analyzed Save time Filling dummy tables accelerates data analysis Analysis

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Preparing the analysis, stage by stage Recoding stage Example: Transform age into age groups Descriptive stage Calculate prevalence or incidence Analytical stage Univariate, stratified and multivariate analysis Prepare empty (dummy) table shells upfront Dichotomize all variables for simple dummy tables –Using the median (e.g., Income > median) –Using a value known to be important (e.g., 200 CD4) Analysis

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Example: Initial stage of the analysis of the study on iodine deficiency according to income Recoding stage Create outcome data with laboratory results Recode income data Dichotomize quantitative income variable to create a “BPL” Yes/ No variable Descriptive stage Calculate prevalence of the thee outcomes Goitre, urinary excretion and salt spot test Adjust confidence intervals for design effect Analysis

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Example: Analytical stage of the analysis of the study on iodine deficiency according to income Univariate analysis Prevalence of three outcomes by age, sex and residence Prevalence of three outcomes by income (potentially examine dose response effect) Stratified analysis Prevalence of three outcomes by income, stratified for age, sex and residence Multivariate analysis Logistic regression model Analysis

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Dummy table for iodine deficiency study (Analytical stage) * Prevalence Prevalence ratio (95% confidence interval) ExposuresExposedUnexposed Female sexXX/XX (XX%) XX (XX-XX) MuslimXX/XX (XX%) XX (XX-XX) Age > medianXX/XX (XX%) XX (XX-XX) Below poverty lineXX/XX (XX%) XX (XX-XX) Schedule casteXX/XX (XX%) XX (XX-XX) * All variables dichotomized for the sake of simplicity

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The analysis plan: A road map to making sense of data 1.Formulate the objectives of the study 2.Choose a design to identify key indicators 3.Identify parameters needed for indicators 4.Prepare the analysis 5.Estimate sample size Sample size

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The analysis plan determines the sample size Choose the study design Cohort, case control or survey Determine the level Descriptive or analytical Common mistake: Designing a descriptive study Trying comparisons for which the sample size is insufficient Sample size

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Sample size for study on iodine deficiency among people below poverty Study design Analytical cross sectional survey Level Analytical Need to: Use prevalence ratio for sample size estimation OR Use prevalence but multiply final sample size by two to allow comparisons Sample size

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Take home messages Clarify precise, focused objectives Choose a design to identify the indicator Know the parameter you want before you think about how to get information about it Know where you go with the analysis The planed analysis drives the data needs and not the reverse Deduct your sample size from all of the above

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Additional resources on analysis plan Dummy tables for field epidemiology Case study on protocol writing (Scrub Typhus in Darjeeling, Volume 2)

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