Using Numbers (iWork) with decimals

you can adjust how many decimal places are displayed by using the increase/decrease decimal places button in the menu bar (as you have noted):

or you can use the cells inspector:

entered 89.65 in cell A1 and have set the number of decimal places to 2

entered 89.65 in cell B1 and have set the number of decimal places to 0

how did you get the value "88.65" into the cell? did you type it in? is it a result of a formula? is so what is the formula?

Here is a screenshot from the Gradebook template:

The formula in cell G4 is =ROUND(SUMPRODUCT(B4:F4,$B$3:$F$3),2)... not really sure why

the bold portion is what is rounding to the nearest whole number:

change G4=SUMPRODUCT(B4:F4,$B$3:$F$3)

then select G4 and fill down

select the cell with the error, then copy the contents of the formula bar and post here:

The formula bar is near the top of the image I am posting and contains (in this case) "=ROUND(SUMPRODUCT(B4:F4,$B$3:$F$3),2)"

if you you triple click the contents of the address bas it will select everything. copy, then paste into this forum.

Hi steph,

Syntax for ROUND is: =ROUND(number,decimal places)

number and decimal places are the two arguments used by ROUND. In the unction, the arguments are separated by a comma.

The original formula in G5 was this: =ROUND(SUMPRODUCT(B5:F5,$B$3:$F$3),2)

Comparing that with the syntax, you'll see these two arguments for ROUND, separated by a comma:

number: SUMPRODUCT(B5:F5,$B$3:$F$3)

decimal places: 2

When you edited this formula, you removed only the function name ROUND, leaving a set of parentheses containing two numbers—SUMPRODUCT(B5:F5,$B$3:$F$3) and 2—separated by a comma and enclosed by parentheses, but with NO instruction telling what to do with this pair of numbers. Numbers doesn't know what you want done with the numbers, do it sends an error message.

Your edit was intended to remove the ROUND function, leaving the unrounded result of SUMPRODUCT. To do that, you need to remove everything except the actual calculation of the number. Your edited formula should look like this in G5:

=SUMPRODUCT(B5:F5,$B$3:$F$3)

Note that column G in the template is formatted to show the result as a percentage, with no places after the decimal. If you want to show results to the nearer hundredth of a percent, you will need to edit the format, using the Inspector, to show two decimal places.

Perhaps a more significant question than "How" to show these results to the nearer hundredth of a percent is "Why" do you want to do this? Is 1/100 of one percent a meaningful amount?

What measurements of the students' work are you taking that you are confident you can distinguish 10000 degrees of achievement?

Here's a visualization to think about:

Consider 20 packages (reams) of paper for your school's photocopier, unwrapped and piled in a single, neat stack. For standard 20 pound bond copy paper, the stack, containing 10000 sheets, will be about a yard high. If the whole stack is considered as "100%", the each sheet of paper is 0.01% of the stack.

Richard Rock's final grade (on the template) is 82.45%, expressed to the nearest hundredth of a percent. How confident are you in the precision of that grade? For the stack of paper, the task to achieve that precision would be to insert a marker (eg. a letter opened or dinner knife) so that exactly 8245 sheets of paper were below the marker, and to do if by estimating or measuring (not counting) the position at which to insert the marker.

Is it possible? Certainly, given a precise enough and accurate enough instrument, and an operator skilled in using that instrument.

Is it repeatable? Probably. Sheets of paper tend to have a pretty consistent thickness, provided you don't change suppliers, finish, or weights, If an accurate measurement can be made to that precision today, then there's a good chance an equally accurate and precise measurement can be made using the stack of new paper available three months from now.

But does this accuracy and precision transfer to the task of measuring student achievement? And if it doesn't, does expressing grades to the nearest hundredth of one percent show a meaningful distinction?

Regards,

Barry

Hi steph,

Aha! The light came on in the shower this morning!

Like other formats, the Percentage format is a way of displaying a number. In this case the number is usually a fraction.

When Sam Student is assigned a grade of 75%, it means that (s)he 75 hundredths of the possible score for a 'perfect' assignment.

75 hundredths may be expressed as a common fraction: 75/100

or in 'standard notation' as a decimal fraction: 0.75

or as a percent: 75%

All three represent the same value.

In Numbers, it's the 'standard notation' (the decimal fraction 0.75) that is used by the ROUND function.

Note that 75%, expressed in 'standard notation,already has (only) 2 places after the decimal.

ROUND(0.75,2) has no effect on that number.

You want to express values as percentages, precise to two decimal places. Example: 66.67%

In standard notation, the same value would be 0.6667

So, since ROUND is written to work with values in standard notation, you need to use 4, not 2 as the last argument.

Revise the formula in column G as shown below:

Original: =ROUND(SUMPRODUCT(B4:F4,$B$3:$F$3),2)

Revised: =ROUND(SUMPRODUCT(B4:F4,$B$3:$F$3),4)

Use the Inspector to change the cell format to Percentage, with two decimal places, then fill the formula down the rest of the column.

Comments and questions on why one would want to claim this precision is meaningful still apply.

Regards,

Barry