The diaphragm is a moderately effective method. For those who use it perfectly, it's around 94% effective. For the typical user (and by typical, we mean the way most end up using it), effectiveness is closer to 84%.
In other words:
Of those women who use the diaphragm exactly as directed, about 6 in 100 will experience an accidental pregnancy during the first year they use this method.
Of those women who do not use the diaphragm exactly as directed, about 12 in 100 will experience an accidental pregnancy during the first year they use this method.