Standard for Intervention on an Appeal From a Sentence
Absent an error in principle, failure to consider a relevant factor, or an overemphasis of the appropriate factors, a court of appeal should only intervene to vary a sentence imposed at trial if the sentence is demonstrably unfit. Parliament explicitly vested sentencing judges with a discretion to determine the appropriate degree and kind of punishment under the Criminal Code.
R. v. M. (C.A.),  1 S.C.R. 500 at para. 90
Except where a sentencing judge makes an error of law or an error in principle that has an impact on the sentence, an appellate court may not vary the sentence unless it is demonstrably unfit.
R. v. Lacasse, 2015 SCC 64,  3 S.C.R. 1089 at para 11.
Sentencing ranges are primarily guidelines, and not hard and fast rules: As a result, a deviation from a sentencing range is not synonymous with an error of law or an error in principle.
R. v. Lacasse, at para 60; see also R. v. Nasogaluak, 2010 SCC 6,  1 S.C.R. 206 at para. 44
Sentencing ranges are nothing more than summaries of the minimum and maximum sentences imposed in the past, which serve in any given case as guides for the application of all the relevant principles and objectives. However, they should not be considered “averages”, let alone straitjackets, but should instead be seen as historical portraits for the use of sentencing judges, who must still exercise their discretion in each case.
There will always be situations that call for a sentence outside a particular range: although ensuring parity in sentencing is in itself a desirable objective, the fact that each crime is committed in unique circumstances by an offender with a unique profile cannot be disregarded. The determination of a just and appropriate sentence is a highly individualized exercise that goes beyond a purely mathematical calculation. It involves a variety of factors that are difficult to define with precision. This is why it may happen that a sentence that, on its face, falls outside a particular range, and that may never have been imposed in the past for a similar crime, is not demonstrably unfit. Once again, everything depends on the gravity of the offence, the offender’s degree of responsibility and the specific circumstances of each case.
R. v. Lacasse, at paras 57, 58