The parity principle means that a sentence should be "similar to sentences imposed on similar offenders for similar offences committed in similar circumstances". 
Given the subjective nature of the sentencing process it is necessary that all sentences "must respect the parity principle".
Offenders being sentenced to the same or similar offence should not have disparate sentences. The sentences should be approximately the same, taking into account the aggravating and mitigating factors unique to the individual. 
The purpose of the parity principle is to ensure fairness "by avoiding disproportionate sentences among convicted persons where, essentially, the same facts and circumstances indicate equivalent or like sentences".
A judge has a responsibility to "impose sentences in line with precedent, noting always that for each offence and each offender some elements are unique." 
To fulfil this requirement sentencing judges must consider the "range" of sentences of "similar offenders for similar offences committed in similar circumstances".
One of the goals of the provincial courts of appeal is to maintain parity in sentence and minimize the differences in sentencing approaches. However, the judges must also be sensitive to the needs of local community.
It has been suggested that courts have a duty "to impose sentences in line with precedent" while still taking into account the uniqueness of the offender and offence.
The parity principle "does not preclude disparity where warranted by the circumstances, because of the principle of proportionality".
Where the judge deviates from the regular range established by case law the judge "should explain how it is in accordance with the principles and objectives and objectives of sentencing."
Given the individualized process of sentencing, "the search for a single appropriate sentence for a similar offender and a similar crime will frequently be a fruitless exercise of academic abstraction".
- see s. 718.2(b) Criminal Code
Sentencing, 7th ed. (Markham: LexisNexis, 2008), at ss.2.21
- Nash v R., 2009 NBCA 7 (CanLII) at para 54
- R v Roche 1990 CanLII 6483 (NL CA), (1990), 84 Nfld. & P.E.I.R. 1, at para 10
- R v W.E., 2010 NLCA 4 (CanLII) at para 33 quoting Clayton C. Ruby, Sentencing, 7th ed. (Markham, ON: LexisNexis, 2008) at page 33:
- R v Oake,  N.J. No. 94 (NLCA)(*no link)
- R v Tuglavina, 2011 NLCA 13 (CanLII),  N.J. No. 25 (NLCA)
Nash at para 54
- R v Rawn, 2012 ONCA 487 (CanLII) at para 18
- R v Knife, 1982 CanLII 2569 (SK QB), (1982), 16 Sask. R. 40 (C.A.) at p. 43
R v Arcand 2010 ABCA 363 (CanLII)
Nasogaluak 2010 SCC 6 (CanLII)
- Nasogaluak, ibid.
R v Oake,  N.J. No. 94 (NLCA)
c.f. R v Day, 2013 BCCA 172 (CanLII), at para 17 (It is “trite law that the sentences imposed in other cases are of limited assistance in determining a fit sentence.”)
R v LM, 2008 SCC 31 (CanLII),  2 SCR 163 at para 36
R v KDH, 2012 ABQB 471 (CanLII) at paras 6-8
- R v Cluney, 2013 NLCA 46 (CanLII)
- R v M(CA),  1 SCR 500, 1996 CanLII 230 (SCC) at para 92
R v Lacasse,  3 SCR 1089, 2015 SCC 64 (CanLII) at para 92
Ranges of Sentence
One of the roles of appellate courts is in "maintaining some degree of uniformity in the sentences imposed for crimes of the same general character" by setting direction of appropriate sentences.
A Court referring to a "range" of sentence can be interpreted in multiples ways. It its descriptive sense, considering a range of sentence is simply a "check" to ensure that the sentence is not unduly harsh or lenient when compared against other cases. It "reflects individual cases, but does not govern them".
In a prescriptive sense, a range is set by an appellate court to instruct lower courts on the "appropriate penalty for a category of crime".
When a range is defined prescriptively, care should be taken as problems may arise in defining the parameters.
Regardless of the sense, ranges are considered "guidelines" and not "hard and fast rules". The judge may go outside the range as long as it conforms to the principles and objectives of sentencing.
Some courts have stated that deviation from the range requires something "special" or "unusual" about the case.
R v Robert, 1996 CanLII 1725 (BC CA) at para 23
R v Johnston, 2011 NLCA 56 (CanLII) at para 36
R v O’Flaherty, 1997 CanLII 14649 (NL CA), (1997), 155 Nfld. & P.E.I.R. 150 (NFCA) at para 20
- Johnston, supra at para 36
Johnston, supra at para 37
R v Stone, 1999 CanLII 688 (SCC),  2 SCR 290 at para 244
Johnson at para 42
R v Murphy, 2001 NLCA 16 (CanLII)
- R v Nasogaluak, 2010 SCC 6 (CanLII),  1 SCR 206 at para 44
- Johnston, supra at para 39
Parity to Co-Accused
In a situation of co-accused, the roles of the parties will be considered as well as their personal circumstances. 
The principle of parity does not require equal sentences for participants in the same offence. Rather, they must "only [be] understandable sentences when examined together."
Where there is disparity between offenders who participated in a "common venture", the judge must show justification.
Where accomplices who commit the crime at the orders of a principal and agree to testify against he principal can be afforded lesser sentences than the principal.
- R v Knife (1982), 16 Sask. R. 40 (C.A.) at p. 43
R v Issa (T.), (1992), 57 O.A.C. 253 at para 9
R v Rawn, 2012 ONCA 487 (CanLII) at para 30
R v Sahota, 2015 ONCA 336 (CanLII) at para 7
- R v Ellahib, 2008 ABCA 281 (CanLII) -- 20 years for the principal and the 16 and 15 year sentences accomplices justified because the principal was the instigator and the accomplices entered guilty pleas and testified against him.