Touchscreen Usability in Short

Events

Contact (touch down): error cancellation a problem!
Loosing contact (touch release): Provides possibility to cancel errors.
There is no analogue to mouse-move events (no comparable feedback possible).
There is no analogue to mouse button - pointing and initiating are combined into one step.

Click: Initiate actions, make selections, specify positions
Double click (cumbersome): Initiate actions, confirm actions
Drag: Define paths (start, goal), draw simple figures or gestures
Duration (long-lasting Touch; do not know applications): Define durations, enter values

Mouse pointer vs. active location (in mouse-based interfaces)

No mouse pointer on touchscreens
- Users do not lose track of their fingers (but they often loose the mouse pointer).
- Users cannot confuse different types of cursors or activated controls.
- There is no analogue to mouse pointer as status display.
Static status display: Can be provided through highlighting (color, frame, 3D look etc.) or animation.
Dynamic status display (Feedback): Can be provided only during the action (and maybe afterwards): The objects themselves have to indicate whether an action is possible/allowed or not! This can be done similar to static activation.

Finger-operated touchscreen: Best in speed and worst in accuracy (Albert, 1982).
Stylus-operated touchscreen: Comparable to a mouse on both speed and accuracy measures (Mack & Lang, 1989).
Initiation of actions: Take-off strategy significantly reduced errors (caused less than half of the errors compared to land-on strategy). Target selection with take-off strategy took longer (about 20%) (Potter, Shneiderman & Weldon, 1988).

Direct: Direct pointing to objects, direct relationship between hand and cursor movement (distance, speed and direction), because the hand is moving on the same surface that the cursor is moving, manipulating objects on the screen is similar to manipulating them in the manual world
Fast (but less precise without pen)
Finger is usable, any pen is usable (usually no cable needed).
No keyboard necessary for applications that need menu selections only -> saves desk space
Suited to: novices, applications for information retrieval, high-use environments.

Low precision (finger): Imprecise positioning, possible problems with eye parallaxis (with pen, too), the finger may be too large for accurate pointing with small objects -> a pen is more accurate.
Hand movements (if used with keyboard): Requires that users move the hand away from the keyboard; a stylus requires also hand movements to take up the pen.
Fatigue: Straining the arm muscles under heavy use (especially if the screen is placed vertically).
Sitting/Standing position: The user has to sit/stand close to the screen.
Dirt: The screen gets dirty from finger prints.
Screen coverage: The user's hand, the finger or the pen may obscure parts of the screen.
Activation: Usually direct activation of the selected function, when the screen is touched; there is no special "activation" button as with a light pen or a mouse.