I am analysing a set of data where I try to predict an outcome (Level of women’s nutrition knowledge; whether it is High or Low) by using certain covariates (demographic characteristics of the sample). I have already done Chi-square analysis and now I am progressing to binary logistic regression.
To avoid an overly complicated presentation of the results by inclusion of a large number of non-significant variables, demographic factors found in Chi-square test to be significantly associated (p<0.05) with women’s knowledge were entered into the logistic regression analysis. These factors are: Prior Pregnancies, Planned pregnancy, Education levels, Household income, First language and Having health and/or nutrition related qualification.
- Dependent variable = Total score of nutrition knowledge of pregnant women; which coded as: a. Low : 0 b. High : 1
a-Prior Pregnancies has 3 levels and coded as:.Tow and more: 0 None: 1 One: 2
b-Planned pregnancy has 2 levels and coded as: N0:0 Yes: 1
c-Education levels has 4 levels and coded as: Some high school or less: 0 High school completed: 1 TAFE: 2 Tertiary education: 3 d-Household income has 3 levels and coded as: < $25000/yr: 0 $25000-50000/yr: 1 >$50000/yr: 2
e-First language has 2 levels and coded as: N0:0 Yes: 1
f-Having health and/or nutrition related qualification has 2 levels and coded as: N0:0 Yes: 1
(As seen above, the levels or class of the independents categorical variables were coded 0 for the lowest interest and 1 for the greatest interest in case of dichotomous variable and so forth for other variables).
My Questions: 1- Which category, the first category (the one of the lowest value) or the last one, should I designate it as the reference category?
2- In case of variable like Age groups which has 4 classes and the first class has the lowest value but the last class has not the highest value; for example the age groups are as the following: Under 20 yrs (has the lowest score in the nutrition knowledge), 20-29 yrs, 30-39 yrs (has the highest score in the nutrition knowledge) and 40 yrs and above (the score of knowledge decreased) How could I choose the category with the highest value to be the reference category when I run the binary logistic regression analysis while it is not the first or the last category?
3- In regard binary logistic regression, which method is better: enter or one of the forward or backward elimination methods? What is the deference between them? Based on what should I choose the method?