Nominal Level: The data can be categorized but cannot be ranked ordered. (Eg. Boys and Girls, Ice-Cream Flavours)
Ordinal Level: The data can be categorised with a natural rank order. There may not be any meaning with regard to the interval between the data class. For example in a Likert Scale, you can categorise responses from strongly agree to strongly disagree but there is no way to determine that the interval between agree and strongly agree is the same interval as between disagree and strongly disagree.
Interval Level: The data can be categorised with a natural rank order. There is a regular interval between the data class. There is no true zero value. (Eg. IQ, PSLE T-Score)
Ratio Level: The data can be categorised with a natural rank order. There is a regular interval between the data class with a true zero value. A true zero value implies the genuine absence of the variable being measured.
What is the difference between using a Histogram & a Bar Chart?
Visually, Bar Charts have a gap between the data groups. This is because the data being categorised are nominal level in nature.
Histograms do not have a gap between the data groups. This is because the data being categorised are ordinal level or higher.