How many people does it take to oust a political leader?

Which are more effective, violent or non-violent protests? And how big does a protest have to be, to drive a political leader out of office? One researcher, who has studied these questions carefully, thinks 3.5% of the population will nearly always succeed.
The Solidarity Movement in Poland in the 1980s, led by the unions; the long-running anti-apartheid movement in South Africa; the overthrow of Serbian President Slobodan Milosevic; the Jasmine Revolution against Tunisia's president, Zine al-Abidine Ben Ali, igniting the so-called Arab Spring…
These are all examples within living memory of popular movements that culminated in substantial political change.
The latest one making the news is in Belarus - where tens of thousands of people have taken to the streets following a disputed election, in which President Alexander Lukashenko claimed victory. The authorities have reacted with brutality; many demonstrators have been arrested and there have been numerous allegations of torture in detention. Despite this, the movement itself has remained largely peaceful.
So is it likely to succeed?
Well, one way to assess this, is to look at history. Which is actually what Harvard political scientist Erica Chenoweth has done.