The principle of stare decisis (to "stand by things decided") refers to the requirement that when a legal issue has been determined and decided, other courts should follow the decision.
The principle is the "glue that holds together the various levels of Canadian courts and it is the principle that elevates the rule of law above the rule of individual judges." It is considered "essential to law" and a "central pillar" to our system of law. It ensures predictability without which differing results would be unjust.
The requirement ensures "consistency, certainty, predictability and sound judicial administration" and the adherence to precedent "enhances the legitimacy and acceptability of judge-made law, and by so doing enhances the appearance of justice".
The principle can be divided into two components. Stare decisis as among the same level of court ("horizontal" stare decisis) and as between different levels of court ("vertical" stare decisis).
The principles does not apply where a decision does not lay out a "substantive rule of law", but simply applies an existing rule to a set of facts.
A statement of a legal principle will amount to an "opinion of the Court" where the principle is accepted by a majority of the Court.
The application of stare decisis does not depend on factors such as the length of the judgement, the extent of the judgement's analysis, or whether the decision is wrong in law.
Canada (Attorney General) v Bedford, 2013 SCC 72 (CanLII) at para 38 ("...the law requires that courts follow and apply authoritative precedents. Indeed, this is the foundational principle upon which the common law relies.)
- R v Hummel, (1987), 36 CCC (3d) 8 at para 7
- R v Arcand, 2010 ABCA 363 (CanLII) at para 182
David Polowin Real Estate Ltd. v. The Dominion of Canada General Insurance Co., 2008 ONCA 703 (CanLII), per Laskin JA
R v Puddicombe, 2013 ONCA 506 (CanLII) at para 65
Canada v Bedford at para 39
- Delta Acceptance Corporation Ltd. v Redman, 1966 CanLII 130 (ON CA)
R v Ibanescu,  2 SCR 400, 2013 SCC 31 (CanLII) at para 1 ("a statement of a legal principle that is accepted by a majority of the Court constitutes the opinion of the Court with respect to that legal principle. This is so even if some of the members of the Court who endorse that legal principle dissent from the majority’s disposition of the appeal. ")
R v Youngpine, 2009 ABCA 89 (CanLII), at para 18
English common law sets out three exceptions of the principles of stare decisis.
- "The court is entitled and bound to decide which of two conflicting decisions of its own it will follow"
- "The court is bound to refuse to follow a decision of its own which, though not expressly overruled, cannot, in its opinion, stand with a decision of the House of Lords";
- "The court is not bound to follow a decision of its own if it is satisfied that the decision was given per incuriam, for example, where a statute or rule having statutory effect which would have affected the decision was not brought the attention of the earlier court"
When considering overruling a precedent from the Supreme Court, the Court should balance the importance of correctness against certainty, considering whether it is more important to maintain certainty with the precedent or ensure correctness by changing it. The Court should be satisfied that there are compelling reasons the precedent should be overruled. 
A trial judge can deviate from precedent where it is based on an argument not raised in the precedent cases or where new legal issues are raised as a product of "significant developments in the law" or if changes "in the circumstances or evidence...fundamentally" changes the "debate".
The threshold for overturning prior judgement is considered "high". As it should not be that the "precedential value of a ...decision ... was thought to expire with the tenure of the particular panel of judges that decided it".
- Young v Bristol Aeroplane Co.,  K.B. 718
Cross and Harris, Precedent in English Law, (4d) (Clarendon Press, Oxford, 1991) at p. 143
Bedford v Canada, 2014 SCC 72 (CanLII) at para 47
- Canada v Craig, 2012 SCC 43 (CanLII) at para 27
- Bedford, supra at para 42 ("In my view, a trial judge can consider and decide arguments based on Charter provisions that were not raised in the earlier case; this constitutes a new legal issue. Similarly, the matter may be revisited if new legal issues are raised as a consequence of significant developments in the law, or if there is a change in the circumstances or evidence that fundamentally shifts the parameters of the debate.")
Bedford, ibid. at para 44
Ontario (Attorney General) v. Fraser, 2011 SCC 20 (CanLII) at para 57
Plourde v Wal-Mart Canada Corp., 2009 SCC 54 (CanLII) at para 13
Horizontal Stare Decisis (Same Level of Court)
Prior decisions of a judge of the same level of court not binding upon a judge. However, where an issue has been decided upon by the same level of court the rule in Re Hansard Spruce Mills Ltd. directs that courts apply "judicial comity" and should follow the same decision unless it is in the interests of justice to do otherwise. 
Re Hansard Spruce Mills Ltd., (1954), 13 W.W.R. 285, 34 C.B.R. 202 (B.C.)(*no link)
Ottaway (Re), 1980 CanLII 401 (BC CA)
A plurality judgement is one were a Court, usually in a panel of 5 or more, decides a case but are unable form a full majority upon the reasons for judgement.
There are no known cases in Canada that address this problem of interpreting the precedential value of the judgement.
In American law, the Courts have developed the "Narrowest Grounds" doctrine which requires inquiry into the ratio decedendi of the case which is to be the narrowest grounds upon which the concurring judgement decided the case. 
In South Africa, there cannot be any binding ratio for a case with less than half of the sitting judges joining the judgement.
Marks v United States 430 U.S. 188 (1977) - original case to develop the Narrowest Grounds doctrine, sometimes call the "Marks rule" as a result
see also Mark Alan Thurmon, When the Court Divides 1992 Duke Law Journal Vol 42
Fellner v Minister of the Interior, 4 S. AFR. L. REP. 523
(App. Div. 1954)
Ratio Decidendi and Obiter Dicta
Each statement of a judge should not be treated as if it were legislation.
Ratio decidendi ("ratio") and obiter dicta ("obiter") are the terms used to distinguish between binding statements of law and commentary within a decision.
Any judicial comment in a decision that forms part of the rationale to reach his decision is considered part of the ratio and is binding upon lower courts.
A comment by a judge in a decision that does not form part of the rationale to reach his decision is considered obiter. Judicial comments that are obiter do not have binding authority on lower courts. Obiter can have persuasive authority, however.
This distinction between ratio and obiter can be fluid.
The degree of weight the obiter has is proportionate to its proximity to the ratio decidendi.
Stated negatively, the statements of a higher court are not binding where the words are "sufficiently tangential to the disposition of the case".
On the outer edge from the ratio consists of "commentary, examples or exposition that are intended to be helpful, but are certainly not 'binding'". 
Courts should begin from the premise that obiter dicta from the Supreme Court is binding.
The purpose of this distinction is to both promote certainty in the law as well as permit "growth and creativity".
- R v Henry, 2005 SCC 76 (CanLII) ("The notion that each phrase in a judgment of this Court should be treated as if enacted in a statute is not supported by the cases and is inconsistent with the basic fundamental principle that the common law develops by experience.")
- e.g. see discussion in Henry
- R v Henry at paras 52-59 ("All obiter do not have, and are not intended to have, the same weight. The weight decreases as one moves from the dispositive ratio decidendi to a wider circle of analysis which is obviously intended for guidance and which should be accepted as authoritative. ")
Canada (Attorney General) v Bedford, 2012 ONCA 186 (CanLII) at para 69 appeal to SCC,  3 SCR 1101, 2013 SCC 72 (CanLII)
R v Prokofiew, 2010 ONCA 423 (CanLII), aff’d 2012 SCC 49 (CanLII)
- R v Puddicombe, 2013 ONCA 506 (CanLII) ("In characterizing obiter from the Supreme Court of Canada, lower courts should begin from the premise that the obiter was binding.")