While most people assume that the best charities are only slightly more effective than average, experts estimate that the best charities are often 100 times more effective.
On the Giving What We Can charity comparisons page, there is a list of concrete examples of how effectiveness varies between charities. For instance, providing deworming treatment is roughly 100 times more effective than merit scholarships are for improving education.

The difference in effectiveness can be even bigger when comparing charities working in a wide variety of causes.

These huge differences in effectiveness provide us with an opportunity to do much more good with our charitable dollars if we choose charities strategically, rather than simply choosing the charities that we are familiar with, or the ones that are most convenient.