In almost all of our experiments, organisms that are all the same length are easier to analyze.  We are often willing to be a little less natural to be much more understandable!  Here are the avida.cfg settings you will need to change from the default:

<table>
<tr>
<td><code>DIVIDE_INS_PROB</code> and <code>DIVIDE_DEL_PROB</code></td>
<td>Insertion and deletion mutations should be turned off; <strong>change these settings to 0.0</strong>.</td>
</tr>
<tr>
<td><code>OFFSPRING_SIZE_RANGE</code></td>
<td>Offspring should be exactly the same size as their parents; <strong>change this setting to 1.0</strong></td>
</tr>
</table>

Although technically not necessary to keep organisms at a fixed length, there is one more option you probably want to change.  One of the main reasons to use fixed length organism is to avoid having to calculate alignments -- you can just line the genomes up.  Without the <code>STERILIZE_UNSTABLE</code> option, organisms may have bizarre implicit mutations which break the assumptions of this easy alignment.

<table>
<tr>
<td><code>STERILIZE_UNSTABLE</code></td>
<td>Sometimes organisms have messed-up reproduction cycles that actually causes them to grow or shrink by copying an instruction multiple times or skipping over it entirely.  These are not insertion and deletion caused by the process of division, but inherent in the organism's copy loop, so they are not turned off when you turn off <code>DIVIDE_INS_PROB</code> and <code>DIVIDE_DEL_PROB</code>.  Avida can, however, check for organisms that are not able to make perfect copies of themselves (i.e. before mutations are imposed) and sterilize them so they do not produce offspring, and this is the setting that turns that checking on.  <strong>Change it to 1</strong>, i.e. "yes".</td>
</tr>
</table>