how to create xpath for the following check box

I am finding it hard to create a relative xpath for a check box . I have attached the code snippet below. Firebug gives me an absolute xpath as - html/body/div[6]/div[2]/form/table[1]/tbody/tr[3]/td[1]/input[1].

But the form changes if there is separate entry and then absolute xpath changes to - html/body/div[6]/div[2]/form/table[1]/tbody/tr[4]/td[1]/input[1] (or tr[5] or any number). I tried to use the relative xpath but the firebug shows the xpath in red colour (so it is invalid)-

.//*[@id='responderSelection'155939'1']

Please help.

<form id="pollGroupForm" method="post" action="/consoleapp/newSurvey.jspx">
<div class="filter"/>
<table class="dataTable">
<tbody>
<tr>
<tr class="alternateRow">
<tr>
<td>
<input id="responderSelection'155939'1" type="checkbox" value="true" name="responderSelection['155939']"/>
<input type="hidden" value="on" name="_responderSelection['155939']"/>
</td>
<td>twoway_2</td>
<td>                                              26161                                                           </td>
<td>
</tr>
</tbody>
</table>
<table>
</form>
</div>
</div>
<div id="footer">