Stata 2, Bivariate

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Stata 2, Bivariate

• Hein Stigum
• Presentation, data and programs at
• http//folk.uio.no/heins/

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Datatypes

• Categorical data
• Nominal married/ single/ divorced
• Ordinal small/ medium/ large
• Numerical data
• Discrete number of children
• Continuous weight

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Data type dictates type of analysis
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Continuous symmetric outcome

• Example
• Birth weight

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Distribution
kdensity weight
drop if weightlt2000 kdensity weight
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Central tendency and dispersion
Mean and standard deviation
Mean with confidence interval
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Compare groups, equal variance?

• Equal

• Not equal

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2 independent samples

• Are birth weights the same for boys and girls?

Density plot
Scatterplot
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2 independent samples test
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K independent samples

• Is birth weight the same over parity?

Density plot
Scatterplot
Equal means? Linear effect? Outliers?
Equal variances?
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K independent samples test
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Continuous by continuous

• Does birth weight depend on gestational age?

Scatterplot
Scatterplot, outlier dropped
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Continuous by continuous tests

• Cut gestational age up in groups, then
  use T-test or ANOVA
• or
• Use linear regression with 1 covariate

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Test situations

• 2 independent samples
• ttest weight, by(sex)
• K independent samples
• oneway weight parity
• By continuous
• regress weight gestAge
• 2 dependent samples (Paired)
• ttest weight_last_year weight_today

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Continuous skewed outcome

• Example
• Number of sexual partners

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Distribution
kdensity partners if partnerslt50
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Central tendency and dispersion
Median and percentiles
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2 independent samples

• Do males and females have the same number of
  partners?

Scatterplot
Density plot
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2 independent samples test
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K independent samples

• Do partners vary with age?

Scatterplot (partnerslt20)
Density plot (partnerslt20)
Scatterplot
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K independent samples test
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Table of tests
Categorical ordered use nonparametric tests
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Categorical data

• Example
• Being bullied

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Frequency and proportion
Frequency
Proportion with CI
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Proportion, confidence interval
xdisease ntotal number

• proportion
• standard error
• confidence interval

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Crosstables
Are boys bullied as much as girls?
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Ordered categories, trend
Does bullied vary with age?
twoway (fpfitci bullied agegr) /// (lfit
bullied agegr)
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Ordered categories, trend
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Table of tests
Categorical ordered use nonparametric tests
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