LABORATORY GRADING POLICIES



LABORATORY GRADING POLICIES

Listed below are a number of factors which are taken into consideration when grading your laboratory reports. Reading this list at the outset of the year and throughout the semester will help you get the best grades on your reports with the fewest misunderstandings.

Experimental Results

The accuracy and precision of your experimental results (see p. 30) count more than any other single factor. They are a reflection of the care with which you did your laboratory work. Each experiment has a different set of expectations which depends on the limitations inherent in the procedure used. For some experiments the results are expected to lie within a few parts per thousand of the correct value in order to have no grade penalty; for others the limits may be 2 or 3 percent. The grading scale used for each experiment is based on several years' experience with student results and, in general, the "correct" result for a given sample is based on the cumulative average of acceptable student results over a period of several years using the same analytical method that you used. The maximum deduction for precision and accuracy will be no more than 30% of the total points for a given experiment.

Significant Figures

The number of significant figures (see p. 31) with which your data are reported, and the number of figures used in reporting mean values, standard deviations, and confidence intervals of the mean (see pp. 39-41) are important with respect to the way you represent your work and the way it will be interpreted by others. The appropriate number of significant figures varies with the experiment; it is independent of the decimal point.

Ever since the ready availability of hand calculators and computers, which commonly carry out all calculations in a continuous sequential manner using at least 8-10 digits, it has made no sense to worry about the rounding off of figures in the intermediate steps of a calculation. In fact, do not round off figures in any intermediate calculations or you might introduce round-off error. The numerical values for all constants used in calculations should be given to the appropriate number of significant figures. When all calculations have been made, apply the rules of significant digits to all results to be reported. Even though 10 digits may be available for the final result of a calculation, it will be your responsibility to choose the correct (rounded) number of digits to represent your analytical results. There will be a grade penalty applied for any measurement or result that is reported to too many or to too few significant figures. The maximum points deducted for significant figure errors will be no more than 10% of the total points for a given experiment.

Calculational Errors

In order that the laboratory assistant may readily assess whether any poor results are caused by poor experimental work or by calculational errors, we ask that you show stepwise calculations (with units) on the back of your report sheets (or sample calculations in the case of multiple determinations). The numerical values for all constants used in calculations, for example, molecular weights, must be shown with your sample calculations. If your calculator has a standard deviation function it is not necessary to show how standard deviation is calculated. Minor calculational errors will be penalized, but in the case of major calculational errors your reports may be returned to you for correction. The corrected results will be graded on the same basis as all others, but a penalty will be levied because of the calculational errors. Presumably this penalty will be much smaller than what would have been levied if the results had been left in an atrocious uncorrected state. The maximum points deducted for calculational errors will be no more than 20% of the total points for a given experiment.

Graphs

In several of the experiments you are asked to graph your results and to obtain analytical information or physical-chemical characteristics from the graph. Some of our major expectations for these graphs are listed below. Failure to meet these expectations will result in the application of a grade penalty. The maximum points deducted for graphing errors will be no more than 20% of the total points for a given experiment.

(a) In some cases you will use a computer to draw your graphs. Graphs generated via any printer are acceptable. Hand-drawn graphs are preferred for the calorimetry and the titration experiments. Put any hand-drawn graphs on good quality mm graph paper or, where appropriate, on semi-log paper; overall size should be 8 1/2" x 11". Suitable graph papers are available at cost at the Chemistry Department Stockroom.

(b) If your graphs are prepared by the computer the units will be scaled automatically so that the majority of the page is used. If they are prepared by hand, however, scale the units so as to use the majority of the graph paper (not just a small portion of it) and employ a scale that is easy to use.

(c) For hand-drawn graphs neatly label the axes to describe what they represent, including the units used. Also provide a caption for the graph, e.g. Titration of an Unknown Acid by a Strong Base. For computer drawn graphs also be sure that the axes are labelled and that a caption is given.

(d) When the origin (0,0) is an important point associated with results it should be included with all other results. This is particularly important when the equation fitting the data is of the form y = mx. (In Microsoft Excel, this is handled by choosing the "constant is zero" button of the regression tool in the data analysis menu, or by setting the intercept equal to zero under options of the add trendline function from the chart menu, as we'll see in the second semester.) When the origin is of no consequence and far away from all other results, a better graphical representation will normally be obtained by omitting it.

(e) For hand-drawn graphs carefully and neatly draw the best smooth curve that represents your work; it may not actually go through any of your experimental points.

(f) If the best curve is supposed to be a straight line, then it should be fit by the method of least squares. (This can be done automatically on the computer, using functions in Excel, again introduced in the second semester.) It is expected that every straight-line plot will be submitted with the slope and y-intercept of the best-fit line, the correlation coefficient, the standard deviation of the residuals, and the 95% confidence intervals for the slope and the y-intercept.

(g) In some instances (such as the endpoint of a titration curve) it may be helpful to make a greatly enlarged section of certain proportions of the curve in order to derive maximum accuracy for the desired result. A good general guide for scale expansion is to use a graph unit which is about twice the size of the instrument unit that is being plotted. For example, if 1 ml on a buret corresponds to an etched-on-the-glass scale length of about 1 cm, then use 2 cm on the graph paper to represent a volume of 1 ml when expanding a titration curve at the endpoint.

Miscellaneous

(a) Yellow sheets. Yellow sheets must be turned in at the end of each laboratory period. Yellow sheets turned in late will receive a 10% penalty. Reports will not be graded until all yellow sheets for a given experiment have been received. The quality of the yellow sheets will be graded with a maximum deduction of 10% of the total points for a given experiment except for "The Chemistry of Five Anions" experiment.

(b) Problems. Some report sheets will contain a problem to solve which deals with the experiment. The maximum deduction for errors in the solution of a problem will be no more than 10% of the total points for a given experiment.

(c) Really lousy reports. No matter how poor the results, how gross the calculations, how flagrantly the number of significant figures has been ignored, or how lousy the graphs, you will receive half credit "for effort" if you do all of the lab work, calculations, and problems, and hand in a laboratory report on time. In general, the class lab average tends to run in the high 70 to low 80 percents year after year. The implication of this is that most people do most of the experiments and the associated calculations quite well.

(d) Uniformity in grading. In order to minimize differences in grading standards that might exist between lab assistants, a list of grade penalties for each kind of error is given to each assistant for each experiment. In addition, the deductions for precision and accuracy are made separately by the instructor in charge of the course, based on pooled student data from all sections and from previous years. Over the years, there has been very little difference in the lab averages between sections.

(e) Lateness. We do not like to take a childish attitude about lateness of reports, but experience has shown that laxness on this one point leads to submission of very poor reports or none at all in some cases. Our policy is that all reports are due in class on the Friday following the scheduled completion of the experiment but in certain announced instances they are due in lab a week after completion at the beginning of the next regular period. Reports that are up to one week late receive a 10% penalty. Reports that are more than one week late receive a 20% penalty. Reports that are more than two weeks late will not be accepted. Exceptions and temporary rearrangement of lab schedules are made for reasonable circumstances such as illness, field trips, scheduled sports events for participants, etc.

51labgra.doc, 13 July 1999

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