The Total column shows the total number of people in that county or town with this surname. For example, there were 155 people called SNEDDEN in West Lothian at the time of the 1881 census.
The Frequency column shows the percentage of people in this county or town with this surname. For example, a frequency of 0.3523 in West Lothian means that 0.3523% of the people in West Lothian on census day were called SNEDDEN.
The Index column shows the relative probability of finding someone called SNEDDEN in this county or town, compared with the probability of finding them anywhere in Britain as a whole. An index of 1 means that if you pick someone at random from this county or town, you have exactly the same probability of picking someone called SNEDDEN as if you picked at random from the whole of the UK. Where the index is higher than 1, then you are more likely to find someone called SNEDDEN here than if you picked from the UK as a whole, and where it's lower then you are less likely. The actual figure shows the level of probability - for example, a figure of 2 would indicate that you are twice as likely to find someone called SNEDDEN here than in the UK as a whole, and 10 would make it ten times as likely. The value of 261.16 in West Lothian means that you are 261.16 times as likely to find someone with the surname of SNEDDEN in West Lothian than you would be in the whole of the UK.