Rounding of Final Value with Sig Figs

In summary, the conversation discusses the proper procedure for calculating the average uncertainty in a lab report, specifically for the index of refraction of a prism. The discussion highlights the importance of understanding how to properly compute the overall uncertainty and the impact of systematic errors. It is recommended to use a weighted average that will be dominated by the measurement with the smallest error.
  • #1
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Homework Statement
Rounding of Final Value with Sig Figs
Relevant Equations
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In my lab report, I average the values of the index of refraction of a prism and I calculated the average uncertainty from the uncertainties in all those measurements. My questions is my final value is 1.585 +/- 0.006

I'm assuming my uncertainty should be rounded to 0.01 but does that mean my value should be rounded to 1.59 or 1.60?
 
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  • #2
The purpose of rounding is to not display meaningless digits. By the same token you want to be sure not to throw away anything meaningful.
To my mind, unless you know your numbers are imprecise for some other reason, your final values should be reported as you initially state them.
 
  • #3
Browntown said:
and I calculated the average uncertainty from the uncertainties in all those measurements.
This is not how the overall uncertainty should be computed.

What exactly are your measurements and errors?
 
  • #4
My apologies for not catching that.
You typically need to do an RMS average which will commonly be dominated by the largest error. Important to understand how this works.
 
  • #5
Thank you for the advice, I looked up RMS and got a better final value.
 
  • #6
hutchphd said:
My apologies for not catching that.
You typically need to do an RMS average which will commonly be dominated by the largest error. Important to understand how this works.
This is not the proper procedure. In the proper procedure the average of the measurements must be a weighted average that will be dominated by the measurement with the smallest error. That error will also typically dominate the overall error (which will also be smaller than all individual errors since you are adding information). This is why I asked OP to post his original underlying data.

One also has to be very mindful of any systematic errors as those do not get better when combining measurements.
 

What is rounding of final value with sig figs?

Rounding of final value with sig figs is a method used to determine the appropriate number of significant figures in a calculated or measured value. It involves adjusting the last digit in a number to reflect the precision of the measurement or calculation.

Why is it important to round final values with sig figs?

Rounding with sig figs is important because it helps to maintain the accuracy and precision of a measurement or calculation. It ensures that the number of digits reported reflects the level of certainty in the measurement or calculation.

How do you round final values with sig figs?

To round with sig figs, you must first identify the last digit that is significant. Then, if the next digit is 5 or greater, you round up the last digit. If the next digit is less than 5, you leave the last digit unchanged. If the next digit is exactly 5, you round up if the last digit is odd and round down if the last digit is even.

Can you round to a certain number of sig figs?

Yes, you can round to a certain number of sig figs by counting the number of significant figures in the original value and then adjusting the last digit accordingly. For example, if you want to round a number to 3 sig figs, you would follow the same steps as rounding with sig figs but stop at the third digit.

What happens to the units when rounding final values with sig figs?

The units of a measurement or calculation should be carried through the rounding process and reported in the final answer. This ensures that the units match the level of precision of the value.

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