<div dir="ltr"><span class="" style="margin:0px;padding:0px;border:0px;outline:0px;font-size:16px;vertical-align:baseline;white-space:pre-wrap;line-height:21px;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif">Hi all,</span><div>
<span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px"><br></span></div><div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px">I’m running some searchlights where (due to task mis-performance and the need to throw out some volumes) there are unbalanced categories, where the exact number of examples/target differ from subject to subject (and from chunk to chunk within subjects!). There aren’t -too- many of these bad volumes, so the categories are generally somewhat still close to 50%:50% category1:category2 (or 67/33 for a separate analysis). </span></div>
<div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px"><br></span></div><div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px">My goal, as before, is to create for each subject an information map where each voxel’s value represents an accuracy-above-chance value for a local sphere. I plan on using these volumes in a top-level analysis in SPM.</span></div>
<div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px"><br></span></div><div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px">I’m wondering:</span></div>
<div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px"><br></span></div><div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px">1. If it’s bad to have slight size-of-categories mismatches between subjects and between targets for any given subject. If so, I guess I could artificially prune some of the “good†volumes from the categories that have greater numbers of examples. However, I’m worried about making my dataset too small.</span></div>
<div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px"><br></span></div><div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px">2. If there is a fairly simple and logical way to calculate the chance-level accuracy rate in a script. </span></div>
<div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px"><br></span></div><div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px">At the moment I use something like this to convert the searchlight-generated error numbers into accuracy maps:</span></div>
<div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px"><br></span></div><div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px"># [10] convert into percent-wise accuracies</span></div>
<div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px">s1_map.samples *= -1</span></div><div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px">s1_map.samples += 1</span></div>
<div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px">s1_map.samples *= 100</span></div><div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px">s1_map.samples -= 50 # for 2-category classification with equal numbers of examples in each category</span></div>
<div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px"><br></span></div><div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px">Ideally, I’d like to automate whatever replaces the “50â€. That said, I’m not sure if the number would always be obvious. (e.g. if I have 100 of one target and 50 of another, would the theoretical chance accuracy rate really be 66.7%? Or some value lower than this?)</span></div>
<div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px"><br></span></div><div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px"><b><u>Completely separately</u></b>, I was wondering if it were possible to add and account for a third row to the chunks/targets text file. One of my potential analyses involves training and testing on completely separate data (i.e. not LOOCV). I was, at first, thinking that I could hijack the “chunks†column for this purpose, giving dataA odd values, dataB even values, and then using an oddevensplitter. However, I still need the -real- chunks for linear detrending, etc. </span></div>
<div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px"><br></span></div><div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px">Thanks for your help!</span></div>
<div><span style="background-color:transparent;white-space:pre-wrap;color:rgb(79,79,79);font-family:'Helvetica Neue',Arial,sans-serif;font-size:13px;line-height:17px">Mike Klein</span></div></div>