Anth 473 / 573: Archaeological Measurement and Lab Analysis

Spring 2003

 

Lab Exercise 1: Precision and Accuracy

 

Due Friday February 7 at the beginning of lab

 

As we discussed in class, the concepts of precision and accuracy are fundamental to evaluating any form of measurement.  These are complementary concepts that describe different aspects of reliability – that is, the ways in which repeated measurements relate to each other.  Precision refers to the closeness of repeated measurements to each other, while accuracy refers to the closeness of measurements to a “true value”. 

 

In this exercise, you will use the concepts of precision and accuracy to evaluate estimates (measurements) of attributes of flakes, ceramics, and soils.  We will be creating several different kinds of estimates and you will compare your estimates to those of others. When the exercise is complete, we will have a series of estimates of attributes that reflect:

1.      multiple measurements of an attribute taken with the same instrument

2.      multiple measurements of an attribute taken with different instruments

3.      multiple measurements of different attributes taken with different instruments

4.      all of the measurements by the students, the TA and the professor

 

The objects from which some attributes will be measured are located at five stations in the lab.  You will make your measurements individually and record those measurements on data sheets that we will provide.  Scott will collect all the data sheets and will make tables that will be posted on the class web site Friday afternoon.  You will need to download the tabulated data in order to complete the lab exercise.

 

You will measure the following attributes:

 

  1. Color.  You will measure the dry and moist colors of a soil sample using a Munsell soil color chart. This involves comparing the sample to a series of standardized color tiles until you have found the closest match. Write the appropriate Munsell color codes on your data sheet.
  2. Soil Texture.  You will estimate the texture of a soil sample using a standard field method, often (colorfully) referred to as a “spit and roll test”.  The procedure involves observing several attributes of the sample when dry and when moist in order to estimate the proportions of sand, silt and clay sized particles.  Scott will demonstrate the method in lab.  Record your texture estimates on the data sheet.  We will compare our estimates to the texture Scott determined using a settling tube method. 
  3. Rim arc and estimated orifice diameter.  You will measure the degrees of arc of two potsherds using a paper template.  Using the same template and sherds, you will also estimate the orifice diameter of the pots they came from.  Record your measurements on the data sheet.
  4. Weight.  You will measure the weight of a potsherd using two instruments: a triple-beam balance and an analytical electronic balance.  Record triple-beam measurements to the nearest tenth of a gram (0.1 g), and electronic measurements to the nearest thousandth of a gram (0.001 g).  Weigh the sherd three times on each scale, and record your measurements on the data sheet.  Do not take your measurements at the same time.  Weigh the sherd on each scale, then move to another station before returning to weigh the sherd for the second time; repeat the process for the third measurement.
  5. Length and Thickness.  Measure the length and thickness of an obsidian flake and a potsherd using both a caliper and a ruler.  Record caliper measures to the nearest tenth of a millimeter (0.1 mm) and ruler measurements to the nearest millimeter.  Again, take three measurements of each attribute on each object and, as above do not take all measurements at the same time.

 

 

Written assignment:

 

Use the standard lab report format as much as possible to answer the following questions.  Please type your answers and attach a copy of the data.  To answer the questions, it may be helpful to construct histograms or calculate means and standard deviations.  For example, the latter will work well for describing variation in estimates of length, width and weight. Any calculations or other data manipulation you perform should also be included, either in the report itself or attached with the data. 

 

  1. For instances where you measured the same attribute using more than one measurement instrument (such as length, thickness, and weight), do you see a difference in the degree of precision represented by different instruments?  Which of the instruments is most precise for a particular estimate?  Which is least precise?  What might account for the differences (or lack of difference) in precision?  Include all instruments in your discussion.

 

  1. Do you note any difference in accuracy for these different instruments?  If there were differences in accuracy, could you determine which is more accurate?  Why or why not?  Include all instruments in your discussion.

 

  1. In measuring the thickness and length of the obsidian flake and potsherd, you used two instruments (caliper and ruler).  For either instrument, does precision vary depending which dimension is being measured?  In other words, does either instrument have a different degree of precision when used to measure length than when used to measure thickness?  Why or why not?  Does precision vary depending on which object is measured?  If so, what accounts for this variation? 

 

  1. How many different Munsell color values did the class identify for the soil sample?  What factors may account for the assignment of different color values to the same sample?  If more than one value each for moist and dry was determined, what does this suggest about the precision of the Munsell chart as a tool for estimating color?  Are there different levels of precision in estimating the color of moist vs. dry samples?  If so, what might account for this?  Can you assess the accuracy of the Munsell charts with this data set? Comment briefly on the usefulness of Munsell charts – does their level of reliability affect how they should be used?  How could you minimize reliability problems if you used Munsell charts in your research? 

 

  1. Did the class generate different estimates of texture for the soil samples?  If so, why?  What does this suggest about the precision of standard field textural determinations?  Can you assess the accuracy of this method?   How could you minimize reliability problems if you use soil textural determinations in your own research? 

 

  1. Looking at the orifice diameter estimates made by the entire class, are there differences in precision between estimates made using the large sherd and the small sherd?  If so, what might account for these differences?  How many sources of bias and/ or measurement error can you identify that are associated with this method of measurement?  How might you minimize these if you used this method in your own research?

 

 

N.B.  These questions are guidelines for your discussion, and you should try to answer each.  DO NOT, however, turn in a list of questions and answers – DO your best to include the answers to the questions in a discussion built around the general lab writeup structure.