7.2 - Recording Results in a Table

What a table question looks like

• Questions may ask you to design a table to record readings, or complete one that's partly filled in.
• A typical question: "A student measures the resistance of a wire at five different lengths, each measured three times. Draw a table the student could use to record the results."

Key rules for table structure

1. Draw one single table with enough columns and rows, and a ruled line under the headings.
2. Head each column with the quantity and its unit, written as "quantity / unit".
3. Record every reading to a consistent precision.

A typical question

A student is going to measure the resistance of a wire at five different lengths. Each length is measured three times. Draw a table the student could use to record the results.

Setting out the table

Which variable goes where

• Put the variable you change (the independent variable) in the first column, with its values going down in order.
• Give the variable you measure (the dependent variable) a column for each repeat, then a final column for the mean.
• Draw the table using a ruler, including a line under the headings.

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Worked example

For an experiment measuring the resistance of five wire lengths with three repeats:

Headings, units, and prefixes

• Write each heading as the quantity, then a slash, then the unit: Length / cm, Resistance / Ω, Time / s.
• Put the unit in the heading once, and leave the numbers below as plain numbers with no units beside them.

Prefixes in headings

• Include prefixes in the heading with the unit:
  • If resistances are in thousands of ohms, head the column Resistance / kΩ and write the values in kilohms.
  • The same rule applies to mA versus A, and ms versus s.
• Any mismatch between the prefix in the heading and the size of the numbers below is treated as a unit error.

Calculated columns

• Any column calculated from raw data (e.g. resistance calculated from voltage and current) needs its own heading with its own unit.
• Use standard unit symbols: do not write "sec" or "secs" for seconds, and do not use "m" for minutes (which means metres).

Filling in the readings

Record the actual measured values, not descriptive labels like "long" or "short".

Consistent precision

• Keep the precision the same all the way down a column. If readings are taken to two decimal places, every entry in that column must be written to two decimal places.
• The precision should match the resolution of the instrument used.

Calculating the mean

• Where repeats are recorded, use the final column for the mean.
• Add the repeats for each row and divide by the number of repeats, then round the mean to the same precision as the raw readings.
• If a repeat is clearly anomalous, exclude it from the mean calculation.

Common exam questions

Question 1

How should the heading of a column that records resistance in ohms be written?

As "Resistance / Ω" - the quantity, then a slash, then the unit - so the values below can be written as plain numbers.

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Question 2

A student writes "20 cm, 40 cm, 60 cm" down the length column of a table. Where should the unit go?

In the heading, once, as "Length / cm". That is the convention markers look for, and it leaves the values below as plain numbers.

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Question 3

A student's resistance readings are all between 1,200 and 4,700 ohms, and the column is headed "Resistance / Ω". What would be better?

Head the column "Resistance / kΩ" and write the values as 1.2 to 4.7, so the prefix in the heading matches the size of the numbers underneath.

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Question 4

An experiment measures the current through a wire at five lengths, with three repeats at each. How many columns should the table have, and what is the last one for?

Five columns: one for the length, three for the three repeats, and a final column for the mean current.

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Question 5

A student calculates a column of resistances from the measured voltages and currents but leaves the new column with no unit. Why does this matter?

Because a column you calculated needs a heading with its unit just as a measured one does; the calculated values are still readings in the table.

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Question 6

One of the three repeats for a length is much higher than the other two. What should you do with it when finding the mean?

Treat it as an anomaly, leave it out of the mean, and find the mean from the two consistent repeats.