My roommate took a home pregnancy test Friday night and it came up positive. She took another one last night and it came up negative. She's going to take another one tomorrow morning. I know she needs to get to a doctor and get a blood test done so we know for sure what's going on, but is it more likely that she is or is not pregnant? I've heard of false positives but never of a false negative, although other sites I've looked at says false negatives are more likely.

False negatives are a lot more common than a false positive. Its rare to get a false positive because when a woman is pregnant her body will release the pregnancy hormone HCG and this is what the home pregnancy test detects in the urine. Sometimes she might not have released high enough levels of this hormone which can result in a false negative.