That depends - every case is different. I didn't have a mammogram, and I found the lump myself - already stage two. It's possible the mammogram wouldn't have caught it, but I often wonder: Suppose it had?
Soon afterward, one of my relatives had a mammogram, and the cancer was caught at Stage 0.
Jwalsh07 found a link to a report on flaws in this study. See post # 78 above.
I saw that and clicked on the link. The font made it unreadable. I have also read that it was a very good study, 25 years long, 900+ subjects. The fact that it was done in Canada is meaningless. The fact that Canada has socialized medicine is meaningless.