Safer User Interfaces: A Case Study in Improving Number Entry

Author(s): Harold Thimbleby
Venue: IEEE Transactions of Software Engineering (Volume: 41, Issue: 7)
Date: 2014

Type of Experiement: Case Study


This paper explores the safety issues of number entry user interfaces and methods of standardization. Number entry is often seen as an elementary process, but it is an integral part of many critical applications. Designers of calculators, spreadsheets, infusion pumps, etc. often dismiss the thought of a formalized user interface and overlook the mass of problems that can be introduced by poor design choices.

These devices were all designed and programmed on independent sets of requirements. For example, suppose a nurse accidentally types in an extra decimal in the calculator, presses the delete button in hopes to remove the extra decimal, and continues typing. The nurse expects his input of “0 . . 5” to result in “0.5”, but the Casio HR-150TEX calculator programmed the delete button to clear the entire input. As a result, the nurse actually administered 5 mL of drugs instead of 0.5 mL, thus killing his patient.

The case study covers an assortment of other problems with number entry user interfaces, such as input field overflow, ambiguous display feedback, and unusual behavior on errors. In addition, the paper suggests rules to minimize these problems in the future, such as rules for number entry and common design decisions. For example, some discuss the placement of the decimal point and the expected functionality of the delete button.