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3.6: Guidelines for Recording and Reporting Data in your ELN

  • Page ID
    476183
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    Always consider uncertainty during an experiment. Do not leave the lab until you are sure that you have a valid strategy for determining errors in all final values.

    Do the following each time you take measurements in lab

    1. Estimate the uncertainty in each experimentally-determined variable. Usually this involves one or more of the following:
      1. Find and record the uncertainties associated with your equipment and instrumentation. (see below for guidelines)
      2. If there are at least three replicate measurements, calculate the standard deviation of the mean of those replicates.
      3. If a value is determined from a function (calcuation), perform a propagation of error analysis.
      4. For values determined from least squares fitting, take note of the errors calculated for the fitted parameters (e.g. slope or intercept of a straight line, and their errors calculated from the lsq script). Always report the uncertainty in the value extracted. If the fitted parameter is used in a function, propagate the error for the calculated value of the function. NOTE: If the data deviate systematically from linearity, a linear regression analysis is meaningless and should not be performed.
      5. If the source or estimation of uncertainty seems more obscure, consult wit your instructors before you leave the lab.
    2. Once you have determined the uncertainties associated with each individual measurement, perform a propagation of error treatment to estimate their combined effect on the uncertainty of the physical quantity of interest.
      **Physical constants such as the Gas Constant, R, may be assumed to be error free.
    3. Report your final result(s), the probable propagated error associated with that result.
    4. If literature values are available, compare your result to the literature: report the percent error or absolute error if appropriate.

    Guidelines

    1. Before you leave the lab, remember to do the following:
      • Make note of the uncertainties in each of the physical quantities you measure. Follow the suggestions below. Consult your TA concerning systematic errors that may arise from instrumental calibration and measurements.
      • Be aware of the consequences of systematic errors, such as starting the clock too late in one of the kinetics experiments.
    2. How to know the uncertainties in a measured value
      (See Characterizing Experimental Error in this manual). This information is often listed right on the device, or is easily obtained from the manufactures statement of equipment specifications. In the rare event that such information is not available, you may assume that single experimentally measured values are uncertain in the last digit by at least ± 0.5 of the smallest unit or measurable digit. For example, if a digital voltmeter reads 5.432 V, and no other information is available, the uncertainty may be estimated to be ± 0.005 V.T
      • Analytical balances: uncertainty is stated on the individual balance and associated with the letter d (for deviation). For example, the label might say d=0.1 mg.
        In the P-chem labs, our balances are usually d= 0.0001 g or d= 0.0002 g. (check it before you walk away from the balance! See Characterizing Experimental Error in this manual)
      • Graduated glassware: uncertainty can be estimated to ± half of the smallest unit measurable.
      • Volumetric pipets and flasks: the error limits can be usually be found right on the pipet or flask itself. If not, refer to a source that lists the tolerance limits for "Class A Glassware". (See Characterizing Experimental Error in this manual)
      • Automatic or digital pipettes: this is typically 0.6-3% of the pipette volume, and depends on the specific pipette used. (See the manufacturer's information or for an estimate Characterizing Experimental Error in this manual)
    3. Treatment of Replicate Measurements
      • If you make a series of replicate measurements of the same quantity, you should report the mean of that quantity together with its precision (standard deviation, variance or 95% confidence limits) calculated by statistical analysis. Unless otherwise requested in the lab manual, use the standard deviation as a measure of precision. If the accepted value of the quantity has been reported in the literature, you might also report the percent error or absolute error as a measure of the accuracy of your measurement(s).
      • If you feel that one of the replicate measurements should be rejected, use the Q-test to justify your decision.
    4. Treatment of a Single Measurement
      • If you make a single measurement of a quantity, give your best estimate of the uncertainty in that measurement, whether random or systematic.
      • If appropriate, report the accuracy of that measurement by comparing it to a literature or expected value.
    5. Fitting
      • If you use least squares fitting to determine the value of any quantity, you must make a decision whether or not to use a weighted least squares regression analysis. In most cases, a non-weighted analysis (using the command lsq in Matlab) will suffice. If you perform a weighted least squares analysis, you will have to decide on what weighting factor, e, to use. If the data deviate systematically from linearity, a linear regression analysis is meaningless, and should not be performed.
      • Outliers may be rejected only on the basis of a Q-test performed on the residuals.
      • The standard error may be used as a measure of the uncertainty in the slope and intercept calculated by linear least squares regression.
      • In most cases, you will use the values of the slope and/or y-intercept to calculate a physical quantity. In either case, you must perform a propagation of error treatment to determine the uncertainty in that physical quantity.
    6. Propagation of Error
      You will use experimentally determined measurements to calculate one or more values from a function. Thus, you must estimate the uncertainty in each measured variable and then propagate that uncertainty through to the final result. In a few simple cases you may take advantage of approximations (shortcuts). Usually however, you will use partial derivatives to calculate the probable propagated error. Physical constants, such as the gas constant, R, may be assumed to be error free.
    7. Reporting the Final Numerical Result(s) in your Notebooks and Reports
      • Each numerical result should be reported together with the associated uncertainty, which is usually the probable propagated error.
      • Where possible, you should report the accuracy of your result by comparison with literature values and calculation of the absolute error.
      • In reporting your results, ensure that the use of significant figures is appropriate to the uncertainty in those results. Use the conventions for significant figures and for rounding off data described earlier in this text. Justify your choices.
    8. Discuss Qualitative Uncertainties in your Notebooks and Reports
      In some cases, it may be difficult to quantitatively estimate the uncertainty in an experimental variable. In this instance, you may be asked to provide a qualitative discussion of the sources of uncertainty in the experiment. This discussion should be as thorough as possible. You should include "reasonable" estimates of the uncertainties in measured parameters together with a discussion of how these uncertainties will affect the final result. Any estimates of uncertainty should be substantiated by hard facts!!

    3.6: Guidelines for Recording and Reporting Data in your ELN is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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