Having thought about the basics, the manuscripts and the possible errors, the final step in the text-critical process is making a decision as to which variant you think is most likely to be original. There are a number of factors and processes that experts use to guide their thinking here.
The evidence for making TC decisions is usually classed as internal or external.
What is the author most likely to have written? Given the flow of their argument, the context, their other writings as well as the broader historical and literary setting all help us to determine what an author is most likely to have written.
What is a scribe most likely to have done? Have they accidentally missed out or added in some text? Might they have simplified or clarified or harmonised?
Sometimes a shorter reading is to be preferred because there is more likelihood that a scribe will have added something in. In other situations a longer reading is preferred because there’s good reason to think a scribe may have missed out some text.
Overall, a text-critic is looking for the one variant which provides the best explanation of how the others arose.
External evidence is like being a jury who not only listens to the account of a number of witnesses (internal evidence) but who must also take into account the reliability of that witness (external evidence). Here we are talking about the date of a manuscript, the qualify of it, the general reliability of it (does it contain lots of errors or additions or harmonisations)? Does this manuscript fit into a particular family or text-type? What are the characteristics of that group?
Also, under external evidence translations, early versions, quotes etc are taken into account – although it has to be kept in mind that these add another level of difficulty because they are derived from other manuscripts and there may be textual issues in even establishing the text of the secondary sources (e.g. LXX).
Making A Decision
Making a final decision about a textual variant is no simple process. It’s not as simple as looking at the different options and choosing one. It is a combination of skill as well as knowledge. Note especially that most weight is given to internal evidence rather than external evidence. The most likely reading is the one that best fits the context of the passage and style of the author.