What Is the Legal Rule About How a Court Today Must Make Decisions Based on Previous Court Decisions?
The Principle of Stare Decisis Requires Judges to Follow the Reasoning of Judges As Was Used In Past Cases In What Are Referred to As Binding Decisions.
A Helpful Guide For How to Understand the Legal Precedent Principle Known As Stare Decisis
In Latin, stare decisis means "To stand by that which is decided". In legal principles, albeit an inaccurate translation, the principle of 'stare decisis' might be most easily understood if thought of as a 'starring decision' that is used when a judge within a current legal case is reviewing a set of facts and issues in comparison to similar past cases for the purpose of receiving guidance and direction while making a ruling for the current legal case. The purpose of the principle of stare decisis is to provide a predictable and consistent manner for deciding similar legal cases and to thereby provide the parties within legal cases with an understanding of what is likely, and thus should be, the expected case decision including the reasoning for that decision.
Is a Judge Supposed to Use Stare Decisis Even If a Prior Similar Case Was Decided At a Same Level of Court?
Generally, Unless There Is Strong Reason to the Contrary a Judge Should Apply the Stare Decisis Principle Even Though the Earlier Decision Was From a Court At the Same Level.
Generally, lower courts are required to follow judicial decisions from higher courts which, being the very basic stare decisis principle as is taught within introductory law classes; however, it can be confusing, even amongst legal professionals and judges, to appreciate whether case decisions should follow reasoning of decisions from courts at the same level.
Per Whitford v. Backman, 2016 CanLII 1308 it appears that a Judge within a lower court should, for consistency and predictability, follow previous decisions even if such is from a past case heard within a court at the same level. In Whitford it was said:
Given this analysis by a court of co-ordinate jurisdiction, the issue before me is whether the doctrine of stare decisis applies. Some courts have held that there can be no stare decisis between judges of the same court only a question of collegiality. However, there is ample authority that stare decisis is a doctrine to be considered by courts of co-ordinate jurisdiction. The obvious rationale for respecting decisions of courts of co-ordinate jurisdiction is that “[i]t is undesirable that a judge sit on appeal from a decision of a judge of co-ordinate authority. To permit such a practice would foster inconsistency and uncertainty respecting decisions made by the same court.” In Wolverine v. R., Wimmer J. stated:
It is true that the doctrine of stare decisis does not absolutely bind a judge of first instance to follow a prior decision of another judge of the same court, but a failure to do so is a disservice to litigants, lawyers and inferior courts who are entitled to see the law as reasonably settled and certain. It is for courts of appeal, not individual judges of equal jurisdiction, to correct judicial errors.
There are only very limited circumstances where stare decisis would not be binding on a court of co-ordinate jurisdiction. In Burroughsford v. Lynch, Goodfellow J. set out the exceptional circumstances. Goodfellow J. stated:
Generally one is bound by the decision of another Justice of the Court when the subject-matter is substantially indistinguishable. I am not bound by any statute or provision in the Judicature Act, R.S.N.S. 1989, c. 240; however, in the interests of predictability and certainty, one should usually follow the decision of another Justice unless one is convinced that the judgment is wrong or there exists strong reason to the contrary. Strong reason to the contrary might be something that indicates the prior decision was given without consideration of a statute or some authority that ought to have been followed, and not simply a strongly held contrary view by the second Justice. One obvious consideration for not being bound by a fellow Justice’s decision is subsequent direction, comment or overruling of that decision by appellate courts.
Goodfellow J. was affirmed by a unanimous Nova Scotia Court of Appeal both as to “the result reached and, in general, for the reasons he gave.” Similar wording for the test of “strong reason to the contrary” is stated in R. v. Northern Electric Co., a decision favoured by the courts, but not referred to in Burroughsford. McRuer C.J.H.C. stated:
I think that “strong reason to the contrary” does not mean a strong argumentative reason appealing to the particular judge, but something that may indicate that the prior decision was given without consideration of a statute or some authority that ought to have been followed. I do not think “strong reason to the contrary” is to be construed according to the flexibility of the mind of the particular judge.
The test of “strong reason to the contrary” is similar to the per incuriam exception to stare decisis. The phrase “per incuriam” literally means “through lack of care.” It refers to a judgment of a court decided without reference to an earlier judgment which is relevant or to a statutory authority which is binding. The court’s attention must be drawn to the relevant authorities. A decision made per incuriam is not binding on courts of co-ordinate jurisdiction. A leading decision is David Polowin Real Estate Ltd. v. The Dominion of Canada General Insurance Co. Laskin J.A., for the court, a five-member panel, stated:
 The insurers rely on the per incuriam exception to stare decisis. This well-recognized exception permits a court to overrule one of its previous decisions if two conditions are met: the panel deciding the earlier case did not advert to judicial or statutory authority binding on it; and had the panel considered this authority, it would have decided the case differently: see Al’s Steak House and Tavern Inc. v. Deloitte & Touche, (1997), 1997 CanLII 2339 (ON CA), 13 C.P.C. (4th) 90 (C.A.); Metro. Toronto v. L.J. McGuinness & Co. Ltd., 1960 CanLII 165 (ON CA),  O.R. 267 (C.A.); Morelle Ltd. v. Wakeling,  1 All E.R. 708, at p. 718.
Accordingly, it appears that the principle of stare decisis should be applied even with cases involving courts of co-ordinate jurisdiction. The exception of course would be where a case is available from a higher court or where the prior case from co-ordinate jurisdiction contains reasoning with a "strong reason to the contrary" and should be ignored.
The stare decisis principle should be applied within judicial decisions even when the prior decision is from a court of co-ordinate jurisdiction, meaning a court at the same level, unless there is an over-ruling higher court decision or the decision from the co-ordinate level is obviously flawed and contains good reason to vary from the prior reasoning.