Rules of Pesak

The following is my translation of the Rema in Hoshen Mishpat 25:2. As you can see, it has significant practical relevance:

I. Following the Greater Scholar

If [the dispute among halakhic decisors] is in a ruling on the permissible and the forbidden (issur ve-heter) and it is a matter of biblical law, one should be strict. If it is a rabbinic law, one should be lenient. However, this is only if the two disputants are equal. We do not rely on the words of someone [relatively] small [in scholarship] against the words of someone greater in wisdom and years, even in a time of need, unless there is a great potential [financial] loss.

II. Following the Majority

Similarly, when there is a solitary view versus a majority, we always follow the majority even if the majority do not agree on a reason but each have their own; since their legal conclusions agree they are called a majority and we follow them.

III. Local Customs

If there was a custom in a city to be lenient because a scholar ruled for them that way, they follow his view. If another scholar comes and forbids what they had permitted, practice this prohibition.

IV. Use of Manuscripts

Any place in which the words of the early scholars are written in a book and famous, and the later scholars disagree with them, as sometimes occurs with the decisors who disagree with the Geonim, we follow the later decisors because the law follows the later scholar subsequent to Abaye and Rava. However, if we sometimes find the responsum of a Gaon that was not recorded in a book and others disagree with it, we need not follow the later scholars because it is possible that had they did not know the words of the Gaon and had they known they would have retracted [their position].

Note that there are subsequent commentaries on this Rema, notably the Shakh here and at the end of Yoreh De'ah 248, the Urim ve-Tumim, the Birkei Yosef and the Arukh Ha-Shulhan. Since this matter is only of practical relevance to a posek, I trust he will be familiar with the sources and not need to rely on a blog. Any posekim who have questions on this subject can feel free to e-mail me.