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 4 edited tags | link edited Jul 6 '16 at 17:32 whuber♦ 215k3434 gold badges471471 silver badges864864 bronze badges 3 added 31 characters in body edited Nov 26 '14 at 17:32 user27241 9811 silver badge88 bronze badges My question seems to be very trivial, but I'm still stuck with it. It's about probability calculations. Suppose you have a number of people that take a glass of water (event), time between 2 events is known (intervals) and events are not independent. I want to calculate probability of a person taking a glass of water in the next 20 minutes (number of minutes can vary) after the last event. I tried to use: Probability = (number of intervals > i and < i + 20 minutes)/number of all intervals where i = 0, 1, 2, 3, ...seconds However, my probability tends to decrease with time, however I expect it to rise. Could someone please advice me on it. My question seems to be very trivial, but I'm still stuck with it. It's about probability calculations. Suppose you have a number of people that take a glass of water (event), time between 2 events is known (intervals). I want to calculate probability of a person taking a glass of water in the next 20 minutes (number of minutes can vary) after the last event. I tried to use: Probability = (number of intervals > i and < i + 20 minutes)/number of all intervals where i = 0, 1, 2, 3, ...seconds However, my probability tends to decrease with time, however I expect it to rise. Could someone please advice me on it. My question seems to be very trivial, but I'm still stuck with it. It's about probability calculations. Suppose you have a number of people that take a glass of water (event), time between 2 events is known (intervals) and events are not independent. I want to calculate probability of a person taking a glass of water in the next 20 minutes (number of minutes can vary) after the last event. I tried to use: Probability = (number of intervals > i and < i + 20 minutes)/number of all intervals where i = 0, 1, 2, 3, ...seconds However, my probability tends to decrease with time, however I expect it to rise. Could someone please advice me on it. 2 edited tags | link edited Nov 26 '14 at 16:54 whuber♦ 215k3434 gold badges471471 silver badges864864 bronze badges 1 asked Nov 26 '14 at 16:49 user27241 9811 silver badge88 bronze badges