6: Simulations in Excel
In this lesson you will design a simulation,
using a spreadsheet.
Before you start this lesson, you should
know how to copy formulas across an
excel spreadsheet. You also need some
printing credit!
By the end of the lesson you should know
how to translate random numbers to
events, and perform lots of trials.
Don’t be afraid to ask for help in
this lesson especially – the
lesson is quite easy, but hard to
learn from a worksheet.
The lesson also works in google
sheets.
Activity
You are running a hairdressers, which usually has 10 customers a day.
There is a
 50% chance the customer will want a cut
 25% a wash and
 25% want their hair dyed.
What is the probability that more than 3 customers in a day want their
hair dyed?
COPY these sentences:
 The DEVICE is a spreadsheet with a random number generator making random
numbers 1-100: randbetween (1, 100)
1-50 will mean a cut
51-75 will mean a wash
76-100 will mean a dye.
 When we make one random number, this simulates one customer. This is AN EVENT.
 For each TRIAL, we need 10 customers (random numbers).
 We need to know if there are more than 3 dyes:
If we got random numbers 17, 84, 53, 28, 12, 91, 58, 100, 62, 64 this would mean
CDWCCDWDWW (this is ONE TRIAL, ie 10 customers).
This is 3 dyes, so the OUTCOME is NO (not more than 3 dyes in a day).
You need these sentences to ACHIEVE.
To set up the spreadsheet, type =randbetween(1, 100) into a cell like this (obviously a
number will appear):
The, copy the formula across 10 cells, and make the columns narrower:
This gives you 10 random numbers. At this point, you could print them and right down what
they mean, but it’s easier to get the computer to do it!
Look at the formula:
 =IF(B2<51, “C”……
This part of the formula says: Write a C (for “Cut”) if the cell B2 is 50 or less (because
50/100 will be cuts). Look – cell B2 is 86 in my line. So it goes onto the next part:
 =IF(B2<51, “C”, IF (B2<76, “W”…..
This part of the formula says: Write a W (for “Wash”) if the cell B2 is 75 or less (because
25/100 ie 51-75 will be washes). It isn’t. So it goes onto the last part:
 =IF(B2<51, “C”, IF (B2<76, “W”, “D”))
… and writes a “D” – the number in B2 is bigger than 75 but less than 100, so it must be a
Dry.
YOU DO NOT NEED TO REMEMBER HOW TO DO THIS BUT YOU MUST UNDERSTAND
WHAT IT MEANS.
Look again:
 =IF(B2<51, “C”, IF (B2<76, “W”, “D”))
Why is B2 there?
Why <51?
Why “C” (the quote marks are important)?
Why 76?
If you don’t know the answer to these, then you need to ask!!
But now it’s easy. Drag this formula across 10 cells, and make them narrow:
I also gave the last column a heading.
Now, select the whole row, and drag it down about 10 lines:
Trial one has the numbers 42, 37, 100, 99, 100, 86, 3, 51, 27, 29 meaning that the 10 events
are C C D D D D C W C C. So the outcome for trial 1 is “YES” – there are more than 3
customers wanting a dye.
Unfortunately, everytime we type in the spreadsheet the events change.
So at this point, print the sheet, and work out the outcomes by hand.1
The probability of more than 3 customers wanting a dye will be the number of YES ÷ the
number of trials (about 10).
In theory, we can quickly do hundreds of trials. But we still have to count the outcomes by
hand! For this exercise, we’ll just do 30, which should fit on one printed page.
Go to 7 Your own simulation
1
You could, instead of printing it, copy and paste it into a word document. This might work in google docs.