Using UnGraph to extract data from image files: verification of reliability and validity

Certain research tasks require extracting data points from graphs and charts. Using 91 graphs that presented results from single-case designs, we investigated whether pairs of coders extract similar data from the same graphs (reliability), and whether the extracted data match numerical descriptions of the graph that the original author may have presented in tables or text (validity). Coders extracted data using the UnGraph computer program. Extraction proved highly reliable over several different kinds of analyses. Coders nearly always extracted identical numbers of data points, and the values they assigned to those data points were nearly identical. Extraction also proved highly valid, with the means of extracted data correlating nearly perfectly with means reported in tables or text and with very few discrepancies in any single case. These results suggest that researchers can use extracted data with a high degree of confidence that they are nearly identical to the original data.