title: graph logistic regression spline, with fixed knots at quartiles in SPSS
body: <p>So I have done my analyses and now I'd like to construct a spline of my logistic regression with 3 knots (at quartile values). On the Y-axis I want odds and on the X-axis I want my linear variable V (between 0 and 500) which I have binned in quartiles in my analysis.
  The outcome table is something like this:</p>
  
  <pre><code>                 B       SE     exp(B)   lowerbound upperbound
   quartiles(1)   0.15    0.329    1.16     0.60       2.18
   quartiles(2)   0.2     0.33     1.22     0.68       2.39
   quartiles(3)   1.2     0.299    3.32     1.70       6.2
  </code></pre>
  
  <p>however I'm not very familiar with SPSS and cannot quite figure it out, do i need to do this manually, and if so would it suffice to use something like:      </p>
  
  <pre><code>     intercept+B1*dummy1*(V-Vquartile1max)+B2*dummy2*(V-Vquartile2max)+b3*dummy3*(V-Vquartile3max)? 
  
       #Not code but expanation: 
       dummy1= (quartile2=1, other quartiles=0)
       dummy2= (quartile3=1, other quartiles=0)
       dummy3= (quartile4=1, other quartiles=0)
  </code></pre>
  
  <p>Does anyone have any suggestions how to do it correctly because the formula I just wrote is way more suitable for linear than logistic regression, and I don't quite know how to fix it.</p>
  
  <p>Kind regards</p>