levers were common. The range and precision of these devices were generally inferior to modern dial type units, with a range of 10/1000 inch to 30/1000 inch, and precision of 1/1000 inch being typical. One common single lever test indicator was the Starrett (No. 64), and those using systems of levers for amplification were made by companies such Starrett (No. 564) and Lufkin (No. 199A), as well as smaller companies like Ideal Tool Co. Devices that could be used as either a lever test indicator or a plunger type were also manufactured by Koch. Digital indicator With the advent of electronics, the clock face (dial) has been replaced in some indicators with digital displays (usually LCDs) and the clockwork has been replaced by linear encoders. Digital indicators have some advantages over their analog predecessors. Many models of digital indicator can record and transmit the data electronically to a computer, through an interface such as RS-232 or USB. This facilitates statistical process control (SPC), because a computer can record the measurement results in a tabular dataset (such as a database table or spreadsheet) and interpret them (by performing statistical analysis on them). This obviates manual recording of long columns of numbers, which not only reduces the risk of the operator introducing errors (such as digit transpositions) but also greatly improves the productivity of the process by freeing the human from time-consuming data recording and copying tasks. Another advantage is that they can be switched between metric and inch units with the press of a button, thus obviating a separate unit conversion step of typing into a calculator or web browser and then typing the results. Contact point (tip) types Plunger (drop) indicator tips On drop indicators, the tip of the probe usually may be interchanged with a range of shapes and sizes depending on the application. The tips typically are attached with either a #4-48 or an M2.5 screw thread. Spherical tips are often used to give point contact. Cylindrical and flat tips are also used as the need arises. Needle-shaped tips allow the tip to enter a small hole or slot. Accessory sets of tips are sold separately and inexpensively, so that even indicators that have no set of tips may be augmented with a new set. Dial test indicator tips Dial test indicators, whose tips swing in an arc rather than plunging linearly, usually have spherical tips. This shape