Why does a PhD take a longer time in some countries? I am talking about computer science. Recently I did survey on my own i mean informally. I find out that there are some researchers who were able to do PhD in same area in 4 year as compare to other researcher who did their PhD in 6 or 6.5 year's in the same research area. Is it depends  upon the duration of the PhD of supervisor or it depends upon the country etc.

Question : Why does a PhD take a longer time in some countries? What factors affect this? Does it affect the research quality? I mean if I am taking more time I should have more quality research.
Many factors that vary systematically between countries may affect time to graduation.

Here is an incomplete list:


In the U.S., grad school starts with studying for an MA degree; in Europe, an MA is a prerequisite to joining grad school
whether or not PhD studies include classes
availability, generosity and length of funding, vs. necessity to earn a living while working on the PhD
incentives to delay graduation vs. incentives and opportunities to join the labor market as soon as possible
quality of supervision and extent of leeway to pursue own interests and potential dead ends.


I'm not aware of any empirical studies testing each factor's relevance.