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Applied biostatistics

Applied biostatistics. Francisco Javier Barón López Dpto. Medicina Preventiva Universidad de Málaga – España baron@uma.es. Multivariate analysis. Generally used to study: the effect of one variable Numerical dichotomous, or qualitative by using multiple binary variables.

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Applied biostatistics

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  1. Applied biostatistics Francisco Javier Barón López Dpto. Medicina Preventiva Universidad de Málaga – España baron@uma.es

  2. Multivariate analysis • Generally used to study: • the effect of one variable • Numerical • dichotomous, or • qualitative by using multiple binary variables. • On another variable • Numerical: Multiple linear regression • Binary: Logistic regression • Controlling for the effect of a few other variables • Control variables • Covariates • Confusion

  3. The usual multivariate model in Health sciences Interesting variable Multivariate model Outcome age Covariates sex … Does [interesting variable] influence [Outcome variable] when [adjusting/controlling/taking into account] covariates1, covariates2,…?

  4. Numerical outcome: Multiple linear regression Interesting variable Multivariate model: Linear regression model Estimate±std.error; p Estimate; CI 95%; p Numericoutcome age Covariates sex • Estimate>0, Increasing effect • Estimate<0, Decreasing effect • Estimate=0, No effect … We are NOT (very) interested in the significance of covariates.

  5. Binary logistic regression Interesting variable Multivariate model: Linear regression model OR; p OR; CI 95%; p Binaryoutcome 0/1 age Covariates sex … • OR>1, Increased risk • OR<1, Decreased risk • OR=1, No effect The estimates now are: Odds Ratios (OR)

  6. Dummy variables Qualitative interesting variable with 3+ levels Multivariate model Outcome age Covariates sex … We must encode the qualitatives non binary variables using only binary variables. How?

  7. Encoding dummy variables Categoria laboral Administativo dummySeguridad=0 dummyDirectivo=0 Seguridad: dummySeguridad=1 dummyDirectivo=0 Directivo: dummySeguridad=0 dummyDirectivo=1

  8. Dummy variables Qualitative interesting variable with 3+ levels Multivariate model Outcome Dummy 1 Multivariate model Dummy 2 Covariates … Outcome Covariates … … Coding qualitative variables using dummy variables

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