OCHRE Wiki - VizWiz: ETC XIX

Visualization: East of the Theater at Corinth (ETC) XIX

By Sandra Schloen, June 2020

Charting, for pottery analysis: East of the Theater at Corinth (ETC) XIX

Over 100 years of excavations at Ancient Corinth by the American School of Classical Studies at Athens has yielded a staggering amount of pottery. OCHRE collaborator, Dr. Kathleen Warner Slane, Professor emerita of Roman Art and Archaeology, at the University of Missouri, Columbia, has pioneered digital methods for analyzing and publishing pottery. In recent years she has focused on the Roman pottery from the area East of the Theater at Corinth, collected during excavations dating from 1981-1990. Prof. Slane has graciously permitted the use of data from her OCHRE project—East of Theater Corinth (ETC) XIX—for some charting examples here.

Disclaimer: Note that these examples are presented using limited selections of data as samples only and we make no comment on its interpretation. For that, stay tuned for Professor Slane's print publication and its digital companion!

Quantifying data using OCHRE Properties

Pottery at Corinth was collected by excavated area in Lots, carefully sorted, and meticulously counted and weighed, with separate counts/weights recorded for different parts of the pottery—Rims, Handles, Bases, Nozzles, and body sherds. This process generated an extensive set of quantitative data which is necessary for producing meaningful charts.

Data such as this is captured in OCHRE using numeric properties; in this case, integer properties for Count variables and decimal properties for Weight variables. Note in the sample OCHRE property sheet provided that the counts and weights of Rims, Handles, Bases, etc. are captured in separate properties.

Fine-grained data is important for careful analysis, but it is often desirable to aggregate quantitative data to determine proportions, distributions, or other kinds of trends. For this we created a derived variable to calculate the sum of the individual counts (RNBHS count), and another to sum the individual weights (RNBHS weight).

Pie chart

The most simple chart is the pie chart, illustrating relative proportions of numeric data. The Format tab of the VizWiz's Content selection pane evaluates the items included in the Set and determines which of the Variables represented among these items are numeric, and therefore appropriate as Data options to chart. Variables that are nominal or ordinal are presented as options for the other dimensions to chart.

For a high-level summary of the pottery by Class, we created a Set containing all the Lots of pottery. In this example we choose the RNBHS Count variable as the source of the numeric data for the chart, and we select the pottery Class as the other dimension, here called the Series, across which the data will be broken out into percentages. We select the Pie chart type and we request that any of the miscellaneous Classes that represent less than 0.25 percent of the total be consolidated into an "Other" class (note the Consolidate below spinner on the Chart options panel). By default, OCHRE will Sum the numeric data to compile the percentages for the pie chart and sort the slices of the resulting pie chart in descending order numerically.

Click the Draw Chart button to produce the chart.

Customizing the chart

To illustrate additional features, in this next example we focus on the collection of Lamps from Corinth, using a query to collect all database items representing lamps that have been catalogued into a Set for analysis. Since we are not comparing Fine ware to Storage jars here, we'll use the RNBHS Weight instead of the Count as the Variable for Data. And since we have restricted our Set to the Class of Lamps, we will choose the pottery Ware as the Series option.

From the Chart Options panel, we choose "Value" instead of "Percentage" as the Data option to display the actual data value, here the Sum of the Weights in kilograms. Since some of these values are small, we set the # decimals to 4 digits. To focus on the main Ware types, we ask OCHRE to consider only the Top-10 wares. Setting the Legend placement to "Bottom" results in a more vertically-oriented display when we refresh by clicking Draw Chart.

XY-Series Charts

An XY-Series chart lets you plot data across multiple dimensions, and OCHRE supports a variety of other chart types, shown here, a simple Bar chart.

On an XY-Chart, the numeric Data variable is plotted on the y-axis and the Series variable is plotted along the x-axis.

But for an XY-Chart you can add an additional dimension, a Group-by option based on a nominal or ordinal variable, to create an outer-level breakdown of the property values. Here we will leave the Ware property as the Series option, but this time we use the pottery Type as the Group-by option. This breaks down, within each Group (Type), the Series (Ware) across the bottom of the chart (the x-axis). By default, OCHRE will create a multi-line bar chart, one line per Series value, for each Group. Due to the number of Lamp Wares this is hard to read, so we chose the Bar chart, stacked option instead. We choose to Sort by Value, descending which provides the visualization in a helpful way for interpretation.

Weight (Data option) of Catalogued Lamps by Ware (Series option), by Type (Group-by option)

Statistics options

OCHRE's Charting support offers a basic set of statistics options, the default behavior being to Sum the values of the Data option variable(s). Other options include Maximum and Minimum, plus Mean, Median, and Mode. For clarity, if the Statistics option is not assigned to the default option, the selected option is added to the y-axis label, e.g. "RNHBS weight (mean)".

Lamps tallied based on Place of manufacture (a nominal property)

Simple counting

If the Statistics option is set to Count there is no need for a quantitative property to Sum or a derived variable to calculate. Instead, choose a Series option for the x-axis and OCHRE will simply tally and plot the number of occurrences of each Value of the selected Variable. If the Data option is set to Percentage, the counts will be displayed as such rather than as absolute values.

(The multi-line graph is illustrated here but the Bar (series), or Stacked bar would also be appropriate options.)

Multiple data options

Let's say that instead of plotting the calculated sum of Rims, Nozzles, Bases, Handles, and Sherds (RNBHS Count or Weight), we want to see each of these Data options plotted separately. Multiple Data options can be selected (instead of a Group-by option) to plot along the additional dimension (e.g. the multiple line/bar/area charts, default or stacked). In this case we select multiple Variables for Data (Base count, Rim count, etc.) a single Series option, and no Group-by option.

Multiple Variables for Data with a single Series option ("Ware"), sorted by Ware "Name, ascending"

Here is the same data as above, but as a bar chart, showing multiple Variables for Data as separate bars (try it as a Stacked bar too).

Base Chart Options on the Set Specifications

By default, OCHRE examines the collection of items to be charted, and tries to determine which variables are valid sources of Data, Series, or Group-by options, based on the Type of variables attested by the items in the collections. But if we save the collection of items as a Set (that is, we are not simply using the quick-view Chart option of the Query Results pane), we have both more control and more options for charting. In particular we can:

Restrict the Variables available for charting options by specifying the Table Columns/Tags list of the Set. In this case we hand-pick the properties to include as options.
Constrain to a particular Value of a Variable by specifying the Table Column to the taxonomic level of a Value, and not just to a Variable. For example, we might constrain "Phase type" to "Building phase" or pottery "Class" to just "Lamps."
Include related information that is not explicit on the core set of items. For example, we might check Periods ON using the option on the Format Specification tab. This makes related (inherited) Periods available for the Chart even if the items are not themselves tagged by Period.

If you choose to use options from the Set specifications, OCHRE will need to generate the Table view of the Set in order to collect the requested column data and optionally any related data. OCHRE will do this behind the scenes, but will inform the user that this needs to happen and request confirmation.

Note that if any Specifications are provided on the Set, OCHRE will, by default, set ON the option to Base Chart Options on the Set Specifications.

Adding the Dimension of Time

OCHRE was designed to handle the dimension of time using the Periods category, and these Periods become a factor in the charting process. The simplest case is when the items being charted are, themselves, assigned to a time Period. For an example of this, see the VizWiz case study of the Taming of the Shrew.

For the ETC XIX project it is not so straightforward. Pottery items are not tagged, themselves, as to Period. It is the Lots within which the pottery items are contextualized that are tagged as to Phase (Period items). The specific pottery groups which are counted and weighed must inherit this information from their context. The data structures needed to support the use of Periods for charting in this example are summarized here:

Valid Periods are itemized in the project's Periods category and organized hierarchically.
Each Period is tagged as to its Period type (here a custom property) , in this case a "Building phase."
Any given counted and weighed pottery item is contextualized within a Lot which is tagged as to its Period (using Period links in the usual way).

ETC XIX Periods for Building phases, assigned to Lots.

Each Period, or "Building phase" in this case, is described by Properties. Because "Period type" = "Building phase" is a property on Period items, it becomes available as a Series or Group-by option on the Set-specification-based charting options.

Because the pottery items are not, themselves, explicitly tagged to Periods, if we want to allow the Period dimension in the chart we need to configure the Set of items to include the needed Period-related properties by specifying Table Columns/Tags along with a Format Specification. Before producing the Chart, OCHRE will generate the Table in the background based on the specification and use the data collected in the Table as the basis for the Chart. That is, when we use the option to Base Chart Options on the Set Specification we can link in information from related items, in this case the Period data.

On the Table Columns/Tags specification, link in the variables (using the Linked Items pane in the usual way for the Taxonomy Category) that you wish to make available to the Chart. Include the variable that describes the Periods that you wish to make available (here, Period type, restricted only to those items with the value "Building phase").

In addition, be sure to check ON the Periods option under the Set's Format Specification. This will ensure that OCHRE will find periods for the current item, or periods inherited by the current item. Be sure to check on the "Check path contexts for groupBy/sortBy property matches" option on the Table Column/Tags panel so that OCHRE will go to the trouble to find inherited periods.

(As an aside ... more on periods)

Tagging the Period items with a property (e.g. "Building phase" above) lets us restrict to those periods assigned to that property value. Otherwise, we will have a time sequence that implicates the full set of potentially nested periods, not just the periods at a specified hierarchical level. That is, without a descriptive Property on the Period items as the basis for selection, OCHRE will define a generic Period option that represents the flattened Period hierarchy. For example, a hypothetical flattened period sequence might be:

Phase 1 ... Phase1a ... Phase1a' ... Phase 1b' ... Phase 1b ... Phase 1c ... Phase 2 ... Phase 2a ... Phase 2b ... Phase 3 ... etc.

But if we tag the top-level phases as Period type = "Archaeological phase" and the nested phases as Period type = "Archaeological sub-phase" we can tell the Table (and hence the Chart) to restrict to the top-level "Archaeological phase" items, giving the more summary sequence:

Phase 1 ... Phase 2 ... Phase 3 ... etc.

We have used this strategy for the "Building phase" periods in this example.

Having provided specifications on the Set, OCHRE will have checked ON the option to:

Base Chart Options on the Set Specifications

This will let OCHRE know to consider only the properties linked into the Table Column/Tags panel, and to honor the options of the Format Specifications (in particular, the search for Periods).

In this example we select the "RNBHS count" as the Data option, the "Ware" as the Series option, and the "Period type" as the Group-by option. Using the Bar chart, stacked option, and sorting by Value, descending gives us the following when we Draw Chart.

Pottery counts, by Ware, by "Building phase" (a property on inherited Period items), sorted by descending value

Use the Save button on the chart menu to save any chart as an image file (PNG format).

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