27.07.2013 Views

The Doctor Rostering Problem - Asser Fahrenholz

The Doctor Rostering Problem - Asser Fahrenholz

The Doctor Rostering Problem - Asser Fahrenholz

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

Chapter 3. <strong>The</strong> model and design 13<br />

3.4 <strong>The</strong> objective function<br />

<strong>The</strong> constraints described in the two previous sections is the complete list of constraints<br />

as the medical practice has described them.<br />

<strong>The</strong> objective function value of a schedule s is the collective sum of the above described<br />

soft constraints and is denoted z(s):<br />

min z(s) = δB0<br />

+ δB2<br />

<br />

i<br />

⎛<br />

yik0<br />

⎝max<br />

k<br />

⎛<br />

+ δB3 ⎝ <br />

+ δB5<br />

+ δB6b<br />

i,j<br />

<br />

i<br />

<br />

i<br />

<br />

+ δB1<br />

i,j∈sa<br />

<br />

i<br />

yik1<br />

<br />

<br />

xijk − min<br />

k<br />

<br />

⎞ <br />

<br />

⎠ + δB4<br />

yijk3<br />

yik5<br />

<br />

yik6b<br />

+ δB6a<br />

<br />

i<br />

<br />

∀k<br />

i<br />

i,j∈sa<br />

yik4<br />

yik6a<br />

xijk<br />

<br />

<br />

⎞<br />

⎠<br />

(Z(S))<br />

An objective function value of 0 means that we have found an optimal solution for any<br />

DRP problem. If the objective function value is positive, it can still be optimal, but<br />

under any circumstances means that one or more soft constraints are invalid. Leaving<br />

the weight, δ, of all the soft constraints equal to 1 means that we can derive from an<br />

objective function value of i.e. 4, that soft constraints are invalid in 4 cases. If we<br />

choose to alter the weights of the rules, the same still applies, but the transparency of<br />

the objective function value decreases.<br />

<strong>The</strong> objective function thus determines the value of solutions for the DRP and is used<br />

by optimal solvers such as GAMS (see chapter 5) and the heuristics in the next chapter,<br />

which will describe how solutions can be found for the DRP.<br />

3.4.1 <strong>Problem</strong> size<br />

<strong>The</strong> amount of variables with 4 shifts per day, 6 doctors and 100 working days:

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!