Now that we have dealt with the use of footnotes in tables in a general way [refer to the earlier post here], let us consider some points of detail.
Formatting the footnote marker: A footnote marker should stand out from the surrounding text, which is why it is common to make the superscript letters or numerals used as footnote markers bold. It is even more helpful not only to make the marker bold but also to italicize it so that it leans away from the text that it qualifies.
Make sure that in setting the marker as a superscript, you are not making the character too small; if it is too small, either use a larger font size or increase the default value for superscripts used by the software package.
If the font that you are using includes proper superscript numerals, use those. You can see the difference for yourself by comparing a superscripted 2 with that obtained by pressing Alt + 0178 (2 versus ²). You will notice that proper superscripts are as dark as the rest of the text, whereas superscripted characters are lighter.
Lastly, check that the column alignment is intact: in a right-aligned column of numbers, for example, the number to which a footnote has been attached should not be pushed to the left to make room for the marker; instead, the marker is "hung," that is, it is placed to the right of the number but outside the column.
Footnote marker at the foot: Some journal publishers do away with the superscript when the marker is reproduced at the foot immediately before the text of the footnote. The ² used as a footnote marker in the table, for example, will be printed as 2 at the foot of the table. These "unsuperscripted" characters are sometimes referred to as "in-line" characters. Observe the practice followed by the target journal and use that.
The text of the footnote begins immediately – with a minimal gap – after the footnote marker. If the superscript form is retained, beginning the text of the footnote with a capital letter can obscure the marker. Again, follow the style of the target journal.
Terminal punctuation: If the text of the footnote is a complete sentence or runs to more than a single sentence, use full stops (periods) as appropriate. If the text is a single word or a phrase or a sentence fragment, no terminal punctuation is required.
More than one footnote in a line: Short footnotes are usually set as a block of text. In other words, the footnotes are "run on" one after the other, separated with a single space, the block being as wide as the lines that make up the normal text or as wide as the table to which the footnotes are attached. However, do not arrange the footnotes in columns even if the target journal uses such a layout. It is then best to start each footnote on a fresh line and leave the layout to the publisher.
The following article gives a more detailed tutorial on how to present your tables and figures for the best possible impact: Tips on effective use of tables and figures in research papers.