Name:____________________ Lab Investigation (SLR) Chemistry Accuracy and Precision Lab Purpose: If you encounter a three part lab the purpose must work for all of them. Lab Objective: Be able to distinguish between the accuracy and precision of estimates made of the measure of several different quantities. Terms: Accuracy = How close a measurement is to some accepted, true value Precision = A term used to describe how close repeated measurements are to each other Procedure I: Estimating Lengths a) Tear a sheet of notebook paper into 8 equal size rectangles b) Without a ruler, draw a free hand line that you estimate to be 5.00 cm long on one of the sheets. Turn the sheet of paper over. c) Repeat drawing of what you estimate to be 5.00 cm long lines on 3 more sheets of paper, each time turning the sheet over so that you are drawing a line without being able to look at your previous estimates. d) Now measure the length of each line with your metric ruler. Enter these measurements in the data table below. Remember that you will have 2 digits to the right of the decimal. e) Hide the first four sheets from sight. Repeat the above process with the remaining 4 pieces of paper, each time turning them over so that you cannot see the other estimates. f) Again measure the actual lengths. g) Enter the data for both trails on the data tables below. Title (Remember to make it descriptive) Measured Length % Error (cm) |5.00 – measure|÷5.00 (Remember .0825343 is 8.25%) Trial 1 Trial 2 Trial 3 Trial 4 Average Title (Remember to make it descriptive) Measured Length (cm) % Error Trial 5 Trial 6 Trial 7 Trial 8 Average Calculations and Questions: (Use complete sentences if applicable) 1. Calculate the average deviation for both trials (see end of sheet for help). ave. dev. trial 1-4: _______________ ave. dev. trial 5-8: _____________ 2. Which of your two sets of estimates was most accurate? How can you justify your answer? 3. Which of the two sets of estimates was most precise? Again, justify your answer. 4. Is the most accurate set also the most precise? Do they have to be? Explain. Name:____________________ Lab Investigation (SLR) Chemistry Procedure 2: Reflex a) Hold a dollar bill so that the bottom edge of the dollar bill is even with your partner’s open index finger and thumb. b) Without telling your partner, drop the dollar bill. Your partner should close his fingers to grab the bill. c) Calculate the distance from the bottom of the bill to the fingers of your partner. d) Repeat this three times and then switch partners. e) Repeat a-e with a meter stick. Compare your data with the class to find an average to use for final data. Procedure 3: Estimating time (You will need a partner for this lab) a) Using whatever timing device you have available (timer, watch, etc.) close your eyes and estimate the passing of 20 seconds. b) When you open your eyes have your lab partner give you your time. c) Record the actual time that passed in the chart below and repeat three more times. d) Complete the calculations for the chart below. Title (Remember to make it descriptive) Measured Length % Error (s) |20.00 – measure|÷20.00 Trial 1 Trial 2 Trial 3 Trial 4 Average Questions 1. Describe your timing device. 2. Did your accuracy improve with practice? 3. Calculate the average deviation of your measured times. To calculate Average (Absolute) Deviation. (you may want to put this in your notes) Take any set of data [7, 9, 5, 7, 2, 6, 4] and numerically sort the set. [2, 4, 5, 6, 7, 7, 9] Find the median (middle number) [6] (for even sets take the two middles and average. Find the absolute value of the difference between the median and each number in the set. |6-2|, |6-4|, |6-5|, |6-6|, |6-7|, |6-7|, |9-7| Æ [4, 2, 1, 0, 1, 1, 2] Find the mean (average) of the set (4+2+1+0+1+1+2) ÷ 7 = 1.571428… or 1.57 This number is the Average Deviation for the set. (The smaller the number the more precise).

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