Title: Building a model
Course: UGQuants (SSPS) > Section: Logistic Regression

Building a model

As with multiple regression, you should use theory to guide how you build a logistic regression model, rather than allowing the computer to select those variables which appear to maximise explanation.  Stay in control by examining the effects of each variable you include by comparing your models step by step.

Let us work through an example based on the British data from the European Social Survey (ESS).  In 2010, it contained a variable mnrgtjb that described respondents’ reaction to the following statement in the self completion part of the questionnaire:

‘Men should have more right to job than women when jobs are scarce'

We have excluded those who failed to complete this part of the survey, to make analysis easier.

The results, summarised in Table 3, show that about 70% of respondents explicitly disagreed with this statement of 'male breadwinner' ideology, while only just over one in ten agreed and a slightly larger proportion neither agreed nor disagreed.

Table 3: Respondents' reaction to statement ‘Men should have more right to job than women when jobs are scarce'

 | N | %
Agree Strongly | 59 | 2.4
Agree | 326 | 13.5
Neither agree nor strongly disagree | 376 | 15.5
Disagree | 1041 | 43.0
Disagree strongly | 591 | 24.4
Don’t know | 29 | 1.2
Total | 2422 | 100

To simplify things, we will divide respondents into those who explicitly disagreed with the statement as ooposed to all others (excluding the small number who said that they did not know, who we will treat as 'missing').

If we simply look at the correlations between these views and other characteristics, a number of patterns that we might expect appear, while some that we might have expected are clearly disproven.  While it is significantly associated with the 5-value indicator, gender, for example, is not significantly correlated with whether the respondent disagrees or not with the statement (it merely influences, how strongly they disagree with women being 1.5 times as likely to strongly disagree than men). However, it did appear that those with more education were more likely to disagree, as were those who were satisfied with their job and the present economic situation, had no religion, tended more to the political right, or felt that they had a secure job.  The biggest effect, however, appeared to be that of age.  Older people, especially the over 60's, were far less likely to disagree with the proposition.  Table 4 summarises these results.

Table 4a: Respondents' reaction to statement ‘Men should have more right to job than women when jobs are scarce' by age group, religion, job security, job satisfaction

 | Age in 3 groups | Doing last 7 days: Paid | Had less security in job, last 3 years | Belonging to particular religion or denomination
18-34 | 35-59 | 60+ | No | Yes | No | Yes | No | Yes
Does not disagree | 22.90% | 23.90% | 51.90% | 41.30% | 23.40% | 24.60% | 19.30% | 25.60% | 39.40%
Disagrees | 77.10% | 76.10% | 48.10% | 58.70% | 76.60% | 75.40% | 80.70% | 74.40% | 60.60%
Phi/Cramer’s V | 0.277 | 0.191 | 0.06 | -0.147
No. of valid cases | 2301 | 2393 | 1232 | 2389

Table 4b: Respondents' reaction to statement ‘Men should have more right to job than women when jobs are scarce' by higher (beyond secondary) education, years spent in full-time education, political orientation and satisfaction with economic situation

 | How satisfied are you in your main job | Years of full-time education | Number of household members | Placement on political left-right scale | Satisfaction with present state of economy in country
(1=extremely dissatisfied to  10=extremely satisfied) | (discrete ratio variable) | (discrete ratio variable) | (values 1= left to 10=right) | (1=extremely dissatisfied to  10=extremely satisfied)
Phi/Cramer’s V | 0.136 | 0.242 | 0.163 | 0.117 | 0.109
No. of valid cases | 1239 | 2384 | 2392 | 2014 | 2337

We could leave matters there, but we would be left with the question: how far are these correlations expressing an indirect effect of age?  It is plausible that those with less secure jobs, or those with religious beliefs, or without higher education, are all older on average than others.  Let's have a look.