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Probability of 2 Sharing Same Birthday

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Submitted By YousufSaya
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Probability of At least 2 People Having the Same Birthday in a group of 25, 50, 75, 100, 500 and 1000
The Simulation has been designed using the computer based program Excel. It is formed to understand the probability of at least 2 people sharing the same birthday in randomly selected groups of 25, 50, 75, 100, 500 and 1000. The simulation is run for 30 iterations for each group that means that the simulation for 25 randomly selected people will be run 30 times and so on.
Assumptions
v The year has 365 days (not considering the leap year). v Every day of the year has an equal probability for a birthday to occur. v The year of the birth is not considered; only the date and the month are considered. v Any other factor influencing the birth such as human behavior or weather etc. is not taken into account. v Twins or triplets are not in same group.

A.
· Open the new excel file, name it Task1, save it on desktop. Take the cursor to cell A1 (the horizontal column is represented by alphabets and vertical rows by numbers and each of the box is called a cell). Write Group 1 and drag it horizontally up to 30 columns the last cell would have Group 10.
· In cell A2 write the formula =RANDBETWEEN (1,365) and enter (we are taking the number of days in a year as we cannot take the dates of months). Then keep the cursor on the lowest right corner and drag until A26.
· Now select the cell A2 and drag it to its right side cell B2 and drag down until B26, similarly do the same process for 10 iterations
· Leave a box or two then write the formula in cell A28 =Mode (A2:A26) and then enter. It will give the day/days; 2 people share the same birthday or #N/A that means no one shares the same birthday. Drag the cursor now from A26 up to 30 Iterations.
· Another way of doing this is to highlight the cells A2 to A26 click conditional formatting from the top bar select highlight cell rules then select duplicate values a small window will appear asking about the color wanted to be applied to duplicates pick a color or just click ok on the preset choice. The duplicates will appear in the chosen color if present. Do the same for rest of the iterations.
Repeat the same process for the group of 50, 75, 100, 500 and 1000 adjusting the formula by putting the respective cell number.
B.
The probability that at least 2 people share the same birthday in randomly selected group of 25, 50, 75, 100, 500 and 1000 people varies, of course the probability increases as the size of group increases and it becomes 100% when there are at least 366 people in group because there are only 365 days in a year so the 366th person will have his/her birthday on the same day with someone. This concept is based on logic but after running the simulation it can be said that it is 100% when there are 75 or more randomly selected people holding all the assumption true.
Probability Table

Size of Groups: Number of Randomly Selected People Per Group|Number of Trials: Number of Groups or Iterations|Number of Successful Trials: Number of Groups with at Least 2 People Sharing the Same Birthday|Experimental Probability That at Least Two People Share the Same Birthday|
25|30|16|53.3%|
50|30|29|96.6%|
75|30|30|100%|
100|30|30|100%|
500|30|30|100%|
1,000|30|30|100%|

There are certain factors that lead to the uncertainty of the results obtained which are as follows:
· Excel is considering that the birthdays are spread out uniformly in 365 days. It does not take into account the human behavior i.e. some people plan to have their babies to be born in a certain month because it’s lucky for them or in US people plan to have their babies before July for school purposes.
· Twins or Triplets if in a same group will impact the results.
· Weather conditions of certain countries restrict them to plan their babies accordingly.
The simulation does not take into account these factors, as some months/days have a higher probability of having at least 2 people share the same birthday; for example, consider May through August in US the probability of people having their birthdays is much higher than rest of the months. The twins and triplets are not considered and can manipulate the results. The weather in certain countries restricts birthday to occur throughout the year with the same probability; for example, if the winters are extremely cold or snowy or seasonal storms make people plan their babies before. Sometimes storms and snow effect the delivery date also.

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