Add new FAQ question

Author: agitter

FAQ.md

@@ -2,8 +2,10 @@
 
 ### Contents
 - [TF activity timing](#tftiming)
+- [TF activity score threshold](#tfthreshold)
 - [Randomization error](#randerror)
 
+
 ## <a id='tftiming'>TF activity timing</a>
 **Q:** How do I identify the time points at which TFs are first active?
 
@@ -20,6 +22,13 @@ To illustrate, suppose you set X = 4.5 in the slider.  Then a TF will be shown t
 
 The ```N.model.activitiesDynamic``` file can also be used to identify all time points at which a TF is active.  In the example above, the TF is active at split nodes with depth 2 and 3 meaning it is active at 15 and 30 minutes.
 
+
+## <a id='tfthreshold'>TF activity score threshold</a>
+**Q:** How do I choose a TF activity score threshold when visualizing active TFs in the DREM display?
+
+**A:** The file ```N.targetsStd```, where *N* is the final iteration of SDREM, can be used to obtain to an activity score threshold that will display all of the TFs that were designated as targets in the SDREM network orientation stage.  The last column in this tab-delimited text file gives the maximum activity score for each active TF.  Therefore, you can find the minimum of these values and use it (or a value slightly less than it) as the threshold.  Take log10 of this activity score to obtain the value of X to set in the DREM Key Transcription Factors slider.
+
+
 ## <a id='randerror'>Randomization error</a>
 **Q:** Why does SDREM throw a randomization error in ```alg.DREMInterface.randomizeBindingPriors```?