Conclusion: Never trust excel, even for a bit complicated statistics!
As a "wanna-be-researcher", of course, I am quite close with data and number. Well, sometimes I use Excel to do some simple things on regards to the data, like drawing a graph, and so on.
But you know what?!
Today my professor and I did several things using excel, and we got real surprised. Imagine... We only did the simple linear trendline, it was only a super simple thing, I mean a real simple thing. The result was y = 29018x - 31865. That was okay, but then when we tested, it went completely wrong!
After that we decided to manually calculate the slope and the intercept. And yes, it was wrong, it should be -318658. We found out that the last digit was missing!
What the heck!