[U] User's Guide

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338 [ U ] 24 Working with dates and times24.7 ReferenceCox, N. J. 2006. Speaking Stata: Time of day. Stata Journal 6: 124–137.
25 Working with categorical data and factor variablesContents25.1 Continuous, categorical, and indicator variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33925.1.1 Converting continuous variables to indicator variables . . . . . . . . . . . . . . . . . 34025.1.2 Converting continuous variables to categorical variables . . . . . . . . . . . . . . . . 34025.2 Estimation with factor variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34225.2.1 Including factor variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34325.2.2 Specifying base levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34425.2.3 Setting base levels permanently . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34525.2.4 Testing significance of a main effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34625.2.5 Specifying indicator (dummy) variables as factor variables . . . . . . . . . . . . . . 34625.2.6 Including interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34725.2.7 Testing significance of interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34925.2.8 Including factorial specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34925.2.9 Including squared terms and polynomials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35025.2.10 Including interactions with continuous variables . . . . . . . . . . . . . . . . . . . . . . 35025.2.11 Parentheses binding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35225.2.12 Including indicators for single levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35325.2.13 Including subgroups of levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35525.2.14 Combining factor variables and time-series operators . . . . . . . . . . . . . . . . . . 35525.2.15 Treatment of empty cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35525.1 Continuous, categorical, and indicator variablesAlthough to Stata a variable is a variable, it is helpful to distinguish among three conceptual types:• A continuous variable measures something. Such a variable might measure a person’s age, height,or weight; a city’s population or land area; or a company’s revenues or costs.• A categorical variable identifies a group to which the thing belongs. You could categorize personsaccording to their race or ethnicity, cities according to their geographic location, or companiesaccording to their industry. Sometimes, categorical variables are stored as strings.• An indicator variable denotes whether something is true. For example, is a person a veteran, doesa city have a mass transit system, or is a company profitable?Indicator variables are a special case of categorical variables. Consider a variable that records aperson’s sex. Examined one way, it is a categorical variable. A categorical variable identifies thegroup to which a thing belongs, and here the thing is a person and the basis for categorization isanatomy. Looked at another way, however, it is an indicator variable. It indicates whether the personis female.We can use the same logic on any categorical variable that divides the data into two groups. It isa categorical variable because it identifies whether an observation is a member of this or that group;it is an indicator variable because it denotes the truth value of the statement “the observation is inthis group”.All indicator variables are categorical variables, but the opposite is not true. A categorical variablemight divide the data into more than two groups. For clarity, let’s reserve the term categorical variable339
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A Stata Press PublicationStataCorp
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iTable of contentsStata basics1 Rea
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iii8.2 The return message for obtai
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v14 Matrix expressions . . . . . .
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vii18.11.6 Writing online help . .
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ix25.2.15 Treatment of empty cells
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Stata basics1 Read this—it will h
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36 [ U ] 2 A brief description of S
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38 [ U ] 2 A brief description of S
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40 [ U ] 3 Resources for learning a
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[ U ] 4.3 help: Stata’s help syst
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52 [ U ] 4 Stata’s help and searc
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58 [ U ] 5 Flavors of Stata5.2 Stat
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60 [ U ] 5 Flavors of StataThe diff
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62 [ U ] 6 Setting the size of memo
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8 Error messages and return codesCo
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[ U ] 8.2 The return message for ob
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76 [ U ] 9 The Break key9.2 Side ef
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78 [ U ] 10 Keyboard usesave is the
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80 [ U ] 10 Keyboard use10.4 Editin
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82 [ U ] 10 Keyboard useAnother met
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11 Language syntaxContents11.1 Over
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[ U ] 11.1 Overview 8711.1.2 by var
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[ U ] 11.1 Overview 89. summarize m
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[ U ] 11.1 Overview 91Example 7Nega
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11.1.7 options[ U ] 11.1 Overview 9
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[ U ] 11.1 Overview 9511.1.8 numlis
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[ U ] 11.2 Abbreviation rules 97by
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[ U ] 11.3 Naming conventions 99The
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[ U ] 11.4 varlists 101You may use
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[ U ] 11.4 varlists 103Consider a v
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[ U ] 11.4 varlists 105Here are som
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[ U ] 11.4 varlists 107You can sele
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[ U ] 11.4 varlists 109. use http:/
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[ U ] 11.5 by varlist: construct 11
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[ U ] 11.5 by varlist: construct 11
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[ U ] 11.6 File-naming conventions
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12 DataContents12.1 Data and datase
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[ U ] 12.2 Numbers 119. display 2/0
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[ U ] 12.2 Numbers 121Technical not
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[ U ] 12.4 Strings 12312.4 StringsA
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[ U ] 12.5 Formats: Controlling how
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[ U ] 12.6 Dataset, variable, and v
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[ U ] 12.8 Characteristics 139You c
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[ U ] 12.10 References 141The Varia
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144 [ U ] 13 Functions and expressi
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162 [ U ] 14 Matrix expressionsHere
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164 [ U ] 14 Matrix expressionsWe c
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166 [ U ] 14 Matrix expressions. ma
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168 [ U ] 14 Matrix expressionsRega
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170 [ U ] 14 Matrix expressionsEith
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172 [ U ] 14 Matrix expressionsThe
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174 [ U ] 14 Matrix expressions7. W
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176 [ U ] 15 Saving and printing ou
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182 [ U ] 16 Do-filesYou then enter
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184 [ U ] 16 Do-files* a sample ana
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186 [ U ] 16 Do-files16.1.4 Error h
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188 [ U ] 16 Do-filesMany users inc
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190 [ U ] 16 Do-files5. If Stata is
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192 [ U ] 16 Do-filesyou could writ
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17 Ado-filesContents17.1 Descriptio
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[ U ] 17.5 Where does Stata look fo
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17.6 How do I install an addition?A
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18 Programming StataContents18.1 De
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[ U ] 18.2 Relationship between a p
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[ U ] 18.3 Macros 205That problem i
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[ U ] 18.3 Macros 207For the right
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[ U ] 18.3 Macros 209To a programme
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[ U ] 18.3 Macros 211Because Stata
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[ U ] 18.3 Macros 213Above we have
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[ U ] 18.3 Macros 21518.3.10 Advanc
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[ U ] 18.4 Program arguments 217You
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[ U ] 18.4 Program arguments 219Let
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[ U ] 18.4 Program arguments 221Equ
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[ U ] 18.4 Program arguments 223pro
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[ U ] 18.4 Program arguments 22518.
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[ U ] 18.7 Temporary objects 22718.
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[ U ] 18.8 Accessing results calcul
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[ U ] 18.9 Accessing results calcul
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[ U ] 18.10 Saving results 2333. Co
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[ U ] 18.10 Saving results 2355. Yo
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[ U ] 18.11 Ado-files 237e(sample)
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[ U ] 18.11 Ado-files 239If you wri
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[ U ] 18.11 Ado-files 241duck is co
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[ U ] 18.11 Ado-files 243begin fvar
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[ U ] 18.11 Ado-files 245begin fvar
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Technical note[ U ] 18.11 Ado-files
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[ U ] 18.11 Ado-files 249{phang}{op
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[ U ] 18.11 Ado-files 2515. Include
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[ U ] 18.13 References 253Technical
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256 [ U ] 19 Immediate commands19.1
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258 [ U ] 19 Immediate commands19.1
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260 [ U ] 20 Estimation and postest
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Page 328 and 329: 316 [ U ] 21 Inputting data21.2 Det
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Page 337 and 338: 22 Combining datasetsYou have two d
Page 339 and 340: 23 Working with stringsContents23.1
Page 341 and 342: [ U ] 23.4 Complex strings 329The p
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Page 345 and 346: [ U ] 24.2 Inputting dates and time
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Page 349: [ U ] 24.6 Converting between date
Page 353 and 354: [ U ] 25.1 Continuous, categorical,
Page 355 and 356: [ U ] 25.2 Estimation with factor v
Page 369 and 370: 26 Overview of Stata estimation com
Page 371 and 372: [ U ] 26.3 ANOVA, ANCOVA, MANOVA, a
Page 373 and 374: [ U ] 26.5 Binary-outcome qualitati
Page 375 and 376: [ U ] 26.9 Exact estimators 363In t
Page 377 and 378: [ U ] 26.14 Models with time-series
Page 379 and 380: [ U ] 26.15 Panel-data models 367xt
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Page 383 and 384: [ U ] 26.21 Multivariate and cluste
Page 385: [ U ] 26.25 Reference 373should try
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388 [ U ] 28 Using the Internet to
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390 Subject and author indexcoeffic
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392 Subject and author indexIif exp
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394 Subject and author indexPiantad
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396 Subject and author indexUnix,ke
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