Accuracy and precision and sig figs Accuracy and

Accuracy and precision and sig figs

Accuracy and precision So much confusion over two words that are used almost interchangeably!!

Precision:

The agreement among several measurements that have been taken in the same way (Precision)

It says how reproducible the measurements are (precision)

Expressed in terms of deviation (precision)

q “tolerance” of an instrument is its degree of precision. For example, the usual platform balance has a tolerance of 0. 1 gram. (precision)

Accuracy: The closeness of a measurement to the accepted value

Here are some bull’s eyes that make it easy to remember accuracy/precision:

Precision? Accuracy?

Precision? Accuracy?

Precision? Accuracy?

Precision? Accuracy?

Sig Figs

Rules for sig figs (taken directly from page 26):

1. Non-zero digits are always significant

2. All final zeros after the decimal point are significant

3. Zeros between two other sig digits are always significant

4. Zeros used solely as placeholders are not significant

5. q The number of sig figs in a measurement is an indication of the precision with which the measurement was taken** **Take note: we make use of this fact in our lab measurements!! Remember to measure as far as there are divisions on the measuring instrument and then estimate one more.

10 20 30 40 50 1 2 3 4 5 What is the precision of these rulers?

Now let’s read measurements. 56. 0 units 10 30 50 70 90 11. 90 units 10 11 12 13 14 3. 75 units 1 2 3 4 5

Read the mass indicated below: 0 0 0. 1 100 10 20 30 40 50 1 2 3 4 5 0. 2 0. 3 0. 4 200 60 70 6 0. 5 7 0. 6 80 90 100 8 9 10 0. 7 0. 8 100 + 80 + 3 +. 470 = 183. 470 g 0. 9 1. 0 g

Arithmetic with sig figs:

Adding and subtracting q Perform the operation q Round off to the least precise value involved

Multiplying and dividing: q Perform the operation q Round off to the least number of sig figs in any factor