Note of Appeal Against Sentence by Dillin Armstrong Against Her Majesty’s Advocate  ScotHC HCJAC_34, June 23, 2021, at paragraph 24:
The principle of comparative justice is an important one. Co-accused persons who have been convicted of the same, or a similar, offence, ought to attract substantially the same sentences (Thomas v HM Advocate  HCJAC 66, LJC (Carloway), delivering the opinion of the court, at para ). If it were otherwise, the sentences, or at least one of them, will be perceived to be unfair. The severity of, or leniency in, a particular sentence on one co-accused must be a factor to be taken into account in sentencing the other co-accused. Any significant difference in sentence ought to be capable of rational explanation (Lambert v Tudhope 1982 SCCR 144 LJC (Wheatley) at 146). That may relate to the backgrounds of the accused, notably their previous convictions (Skilling v McLeod 1987 SCCR 245 LJC (Ross), delivering the opinion of the court, at 248). It may be that the roles played by each co-accused were radically different, where the conviction is based on art and part guilt (Cosgrove v HM Advocate 2008 JC 102, Lord Macfadyen, delivering the opinion of the court, at para ; although cf Simpkins v HM Advocate 1985 SCCR 30, LJG (Emslie) at 35). It may also be that one or more of the co-accused may be significantly younger than the others. The exercise involves scrutinising all of the sentences in order to see that the comparison principle has been fully applied.