I'm trying to install InStat 3a on a Mac with OSX. The installer stops, saying that it requires 4096K of physical memory, but my computer has several GB of memory.

The installer for InStat 3a doesn't work properly on computers with > 2GB of RAM, and gives a silly error message about too little RAM. Request that we email you the installer for InStat 3b. Include  your serial number with your request which you should submit your request at help.graphpad.com.