This applies to perceptual as well as a priori beliefs. They say we do not know whether an external world exists. Skeptics do not doubt their own existence. Other people are external objects, just like all other physical objects are. A skeptic does not grant you knowledge of other people and then say 'we' can't know of the existence of external objects.
Sense data exist in the individual mind.
No two people can have the same sense datum. Sense data exist only when being perceived. Sense data vary depending on observer's condition. The objects that you see change depending on your perspective. Physical objects do not change depending on your perspective.