2. A student finds their standard deviation in Part III to be very small for the volume of water dispensed by the pipette. Does this mean that the student has little random or systematic error? Are the student's results precise? Are they accurate? Explain.
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This means the student's results are precise. Show more…
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Experimental measurements usually have some uncertainty which can be introduced by random error or by systematic error (or bias). Random errors show up as differences in the measurement values, and the extent to which a set of measurements agree with one another is called the precision of the measurements. Systematic errors affect the accuracy of the measurement, and must be analyzed by calibrating the measurement with samples of a known value. Statistics presents a variety of ways of describing and analyzing these errors, and determining the confidence one can have that the "true value" lies within a certain range of values. The concept of standard deviation is a way of treating random errors that assumes the distribution of the total population of measurements follows a "normal" distribution curve (sometimes called a "bell curve"). A set of measurements is usually only a sample of the total "population" of measurements that might be made, so the formulas in the right hand column above apply. You are asked to calibrate a 10-mL volumetric pipette by weighing to the nearest 0.1 mg the mass of water delivered by the pipette. You weigh six samples of water delivered by the pipette and convert the mass of each to volume by multiplying by the volume of 1.0000 g of water at 25°C (1.0040 mL). Following are your measurements: 9.9820 mL, 10.0460 mL, 10.0520 mL, 10.0210 mL, 9.9620 mL, 10.0080 mL. Calculate the following statistical measures for this data: Mean (x), Variance (s^2), Standard Deviation (s).
Adi S.
Four students delivered water into a beaker using 25-mL pipets (tolerance = ±0.03 mL). The students' mean volume and standard deviation results were as follows: Student A: 25.02 ± 0.58 mL Student B: 24.99 ± 0.02 mL Student C: 25.32 ± 0.01 mL Student D: 24.77 ± 0.25 mL 1. Which of the students had poor accuracy but good precision? a) A b) B c) C d) D 2. Which of the students had good accuracy and good precision? a) A b) B c) C d) D 3. Which of the students had poor accuracy and poor precision? a) A b) B c) C d) D 4. Which of the students had good accuracy but poor precision? a) A b) B c) C d) D
Madhur L.
During transfer of liquids using a micropipette: 1- What would you and/or your professor consider an acceptable standard deviation for your pipette? 2- Would you consider your pipettes accurate? Why? 3- In this experiment you are working with one clear assumption regarding your balance that may or may not be appropriate. What could be this assumption? 4- What would it mean if the mean is very close to the expected value but your standard deviation is high?
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