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I'm reading a paper which calculates the probability of an electric car re-charging. Data has been recorded from a car. There are 2 types of events, a charge event and a Trip (journey) event. The State of charge of the battery (SOC) was recorded at the start of each event. For example a journey started with 50% SOC or charge event started with a 10% SOC.

The charge shows (1) the frequency of trip and charge events for each SOC and (2) The author calculates the 'probability of charging' (the line on the graph) for each state of charge bin according to the formula below.

My question is,

(1) Does that formula make sense? Why would you divide by the number of trip events?

(2) Does the text and formula contradict each other as the text says

"The probability of a vehicle undergoing a charge event is calculated by proportion ofthe total number ofcharge events at each starting State of Charge (SoC) as a function of all charging events. "

but the formula divides by the number of trip events.

Appreciate any comments

Thank you

https://dl.dropbox.com/u/54057365/All/prob.JPG [Broken]

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# Simple probability question

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