A "reasonable suspicion" refers to the "possibility of uncovering criminality, not the probability of doing so".
A "suspicion" refers to an expectation that a person is "possibly engaged in some criminal activity."
The suspicion must be reasonable, which requires "more than a mere suspicion and something less than a belief based upon reasonable and probable grounds". It must be supported by factual elements that can be independently assessed.
Reasonable suspicion is a standard lower than "reasonable and probable grounds". The main distinction is "merely the degree of probability demonstrating that a person is involved in criminal activity, not the existence of objectively ascertainable facts".
It follows that the "degree of reliability and the amount of information to establish that lower threshold is lower" than RPG.
The presumption of reasonable suspicion should "not be disturbed unless it is unreasonable or not rationally capable of supporting an inference of suspicion." 
In evaluating police decision-making, the reviewing judge must "take into account that the police at the scene are often required to make quick decisions based on available information, some of which may not be complete or exact, in situations that are rapidly changing and potentially volatile."
The judge should also account for an officer's experience in the particular type of investigation. The officer's experience may allow him to draw inferences and deductions that regular people would fail to make.
- R v Navales, 2014 ABCA 70 (CanLII), at paras 18
- R v Kang‑Brown 2008 SCC 18 (CanLII) at para 75
- Kang-Brown, ibid. at para 75
- R v Xuan Nguyen, 2013 SKQB 36 (CanLII) at para 27
- R v Kang‑Brown, supra at para 75
- Xuan Nguyen, supra at para 27
Xuan Nguyen, supra at para 29 citing R v Gunn
See R v Gunn, 2012 SKCA 80 (CanLII), 399 Sask.R. 170, at paras 15‑23
Xuan Nguyen, supra at para 30
Xuan Nguyen, supra at para 31
Reasonable Suspicion Test
A reasonable suspicion lies between a mere suspicion and reasonable and probable grounds. 
Reasonable suspicion does not require that it be the only possibility, but merely one possible conclusion based on supported facts.
Reasonable suspicion can rely on information that is less reliable than for establishing "reasonable and probable grounds".
Reasonable suspicion must include objective reasonableness. 
The assessment is based on the totality of the evidence-all surrounding circumstances-rather than piece by piece consideration of whether the evidence is consistent with the factors.  The inquiry must consider "the constellation of objectively discernible facts said to give the investigating officer reasonable cause to suspect that the individual is involved in criminal activity".
Where suspicion deals with possibilities rather than probabilities it necessarily means that it is possible to reasonably suspect that innocent people are involved in crime.
It also means that reasonable suspicion does not need to be the only inference from a particular constellation of factors.
The obligation of police to consider all factors does not require them to make inquiry into exculpatory factors or ruel out innocent explanations.
The evidence forming reasonable suspicion need to indicate the "possibility of criminal behavour". The evidence itself need not include unlawful behaviour or evidence of a specific criminal act.
The suspicion must be "sufficiently particularized" as an overly "generalized suspicion" would capture too many innocent persons. Accordingly, factors that may "go both ways" are not sufficient on their own to support reasonable suspicion. Such factors do not preclude reasonable suspicion arising when they are part of the constellation of factors.
Any factors that are favourable or unfavourable (including "exculpatory, neutral or equivocal") must still be considered as part of the totality of circumstances.
Standard of Review
Determination of whether factors amount to a reasonable suspicion is question of law reviewable on a standard of correctness.
R v Kang-Brown, 2008 SCC 18 (CanLII),  1 SCR 456, at para 75
see R v Monteyne, 2008 SKPC 20 (CanLII) (re suspicion)
R v Donald,  S.J. No. 564, 2010 SKPC 123 (CanLII) at para 18
- R v Chipchar, 2009 ABQB 562 (CanLII), (2009) AJ No 1058
- R v Kang-Brown, 2008 SCC 18 (CanLII),  1 SCR 456,  SCJ No 18 at para 75 at 79
- R v Smith, 2011 SKPC 149 (CanLII),  S.J. No. 650 (Sask. Prov. Ct.) at para 44
R v Ajula 2011 ONCJ 10 (CanLII)
R v Todd, 2007 BCCA 176 (CanLII)
R v Wong, 2001 BCCA 13 (CanLII)
R v Usher, 2011 BCCA 271 (CanLII)
R v Nahorniak, 2010 SKPC 68 (CanLII) at para 23
R v Cuthbertson, 2003 ABPC 83 (CanLII) at 46
R v Navales, 2014 ABCA 70 (CanLII) at paras 19
R v Chehil, 2013 SCC 49 (CanLII) at paras 29, 31
Chehil, ibid. at para 28
Chehil, ibid. at para 32
Chehil, ibid. at para 34
Chehil, ibid. at para 35
Navales, supra at para 19
Chehil, supra at paras 30 to 31
Navales, supra at para 19
Chehil, supra at para 31
- Chehil, supra at para 31
Navales, supra at para 19
Chehil, supra at para 33
R v Wunderlich, 2014 ABCA 94 (CanLII), at para 8
R v MacKenzie, 2013 SCC 50 (CanLII), at para 54
The subjective belief for the demand is a question of fact.
The judge is permitted to consider the police officer's training and personal experience in determining objective reasonableness.However, the evidence of a police officer is no less worthy of scrutiny.
The officer himself cannot dictate what is reasonable and what is not. 
Observations of a suspect running away from police, on its own, is not enough to allow for police to detain or search.
R v Rajaratnam, 2006 ABCA 333 (CanLII) at para 25
R v Juan, 2007 BCCA 351 (CanLII) at paras 18-21, 222 CCC (3d) 289.
R v MacKenzie, 2011 SKCA 64 (CanLII)
- R v Payette, 2010 BCCA 392 (CanLII) at para 29
- e.g. R v N.(N.), 2009 ONCJ 508 (CanLII) - police see suspect running away from them in high crime area, no reason to believe crime had occurred
Compared to Reasonable Grounds
A police officer may demand that a person who is found in care and control or in operation of a vehicle undergo a roadside screening test for alcohol. The officer must have "reasonable grounds" to suspect (or "reasonable suspicion") that alcohol is in their body to make a roadside test demand. The quantity of alcohol and the level of impairment is irrelevant.
Burden of Proof
The burden is on the Crown to prove that there were grounds to administer the test on a balance of probabilities.
Failure to For Grounds
Where an officer administers an ASD without reasonable suspicion, a Charter violation under s. 8 and 9 result. 
Sufficiency of Grounds
The standard required for the ASD test under 254(3)(a)(i) is not a “standard of proof” as understood in judicial proceedings.
An admission of having consumed alcohol without mention of when and how much can be considered sufficient to form a reasonable suspicion.
Compelled Statements or Participation
Evidence that was obtained from the accused through his compelled participation can only be used to establish grounds of suspicion and cannot be used to incriminate the accused.
R v Maslanko, 2011 ABPC 202 (CanLII)
R v Haas, 2005 CanLII 26440 (ON CA)
R v Church, 2008 BCSC 686 (CanLII) at para 6
R v Gaudaur, 2008 BCSC 981 (CanLII) at paras 40-42
R v Lemma, 2011 ABPC 312 (CanLII) (no grounds)
R v Beechinor, 2004 SKPC 49 (CanLII),  S.J. No. 187 (SKPC)
e.g. R v Zoravkovic,  O.J. No. 1010 (Ont. C.J.), 1998 CanLII 3202 (ON CA)
R v Hendel,  O.J. No. 2849 (Ont. C.J.)(*no CanLII links)
R v Smith,  O.J. No. 3677 (Ont. C.J.)(*no CanLII links)
R v O’Flanagan,  S.J. No. 55, 2009 SKPC 14 (CanLII)
R v Thompson,  S.J. No. 240, 2003 SKPC 56 (CanLII)
- R v Loewen, 2010 ABCA 255 (CanLII) at para 13
R v Flight, 2014 ABCA 185 (CanLII) at para 39 to 61
- R v Orbanski; R v Elias,  2 SCR 3, 2005 SCC 37 (CanLII), at para 58
Reasonable Suspicion Test
The standard only requires a belief on the presence of alcohol and not the amount consumed, the effects or degree of impairment.
The suspicion is of the "consumption alone and not its amount or behavioural consequences".
Multiple Officers Involved
The officer who makes the demand does not need to be the officer who had initial contact with the accused. However, the officer making the demand must be the one who formed a reasonable suspicion that the accused has alcohol in his system. This basis can be based on information received from another officer.
The requirement of considering the totality of circumstances requires considering not only evidence suggestive of alcohol in the body but also evidence suggesting otherwise as well as the absence of evidence.
The objectively verifiable evidence should not be dissected and individually tested.
Source of Alcohol
An officer should be able to describe the origin of the alcoholic smell, by indicating whether other people were in the car, or otherwise suggesting where the source of the odor could be from.
It is not necessary that the crown eliminate all other possible sources of the odor of alcohol to form reasonable suspicion.
Requirements for Reasonable Suspicion
For the peace officer to form reasonable suspicion, the officer must:
- subjectively and honestly suspect the driver who had alcohol in his body; and,
- The subject of suspicion must be based on "objectively verifiable circumstances"
Reviewing the grounds requires the application of the Mackenzie test: "what a reasonable person, standing in the shoes of the investigating police officer and aware of all the objectively verifiable evidence, reasonably suspect the driver had alcohol in his or her body?"
There should not simply be signs of consumption of alcohol, but also signs of impaired driving skills. Once both are established, the analysis must consider the degree of each and the totality of the circumstances. 
Impairment to driving skills requires that on an objective basis there is impairment “such as coordination, comprehension and a poor (but not simply illegal) driving pattern.” If any one of these is found as well as evidence of alcohol consumption, then there is an objective basis to conclude the driver’s ability to drive is impaired by alcohol.
Alternate explanations or imprecise descriptions do little to reduce the value of the observations. This is because the observations to not need to meet a formal burden of proof. Also, most any signs can be explained by something other than alcohol.
An admission of "two drinks", without any indication of the time of these drinks, was enough to support a reasonable suspicion.
An admission of “two drinks” is not relevant to establishing impairment. It is only where there is an admission of many drinks, that the inference of impairment can be made.
Evidence of an amount of consumption can also be enough to create a reasonable suspicion.
Grounds should include more than simply an admission of the accused that they had drank alcohol within the past three hours. 
- R v Baltzer, 2011 ABQB 84 (CanLII) at paras 40 to 41
e.g. R v Spiry, 2010 ABPC 61 (CanLII), at para 19 - evidence of consumption only is not likely sufficient to form reasonable grounds
- R v Baltzer at para 38
- R v Baltzer 2011 ABQB 84 (CanLII) at 36-37
- R v Kimmel, 2008 ABQB 594 (CanLII)
R v Thomas, 2008 ABQB 610 (CanLII)
R v Ross, 2011 ABPC 173 (CanLII)
c.f. R v Dyer, 2007 ABPC 116 (CanLII)
- R v Baltzer at 36-37
- R v Gilroy, 1987 ABCA 185 (CanLII), (1987), 79 A.R. 318 (C.A.), leave to appeal to S.C.C. refused
R v Stauch, 2007 ABQB 85 (CanLII)
R v Mowat, 2010 BCPC 430 (CanLII)
R v Baker, 2004 ABPC 218 (CanLII),  A.J. No. 1355
R v Hnetka, 2007 ABPC 197 (CanLII),  A.J. No. 806
R v Klontz, 2007 ABPC 311 (CanLII),  A.J. No. 1283
R v Hemery, 2008 ABPC 209 (CanLII)
The grounds need not be proven a on balance of probabilities.
To be reasonable it does not need to be the only conclusion derived from the circumstances. The court considers whether a reasonable person in the circumstances would have a suspicion that the person was impaired by alcohol.
The smell of alcohol on the breath of the driver, by itself, is sufficient to support the suspicion that the driver was operating the vehicle while impaired. Or the suspicion that there was alcohol in their body.
However, the smell of acohol in the vehicle does satisfy the objective grounds to use the screening device.
It is not necessary to consider the timing at which the suspected alcoholic beverage was consumed.
The lack of evidence of consumption can weigh against the formation of a reasonable suspicion.
An officer need not spell out his subjective suspicion that there is alcohol in the accused’ body using the words of s. 254(2). The court may infer the suspicion based on all of the evidence.
It is generally considered that the “point in time” for the issue of reasonable grounds is at the time of the arrest. Certain cases have concerned the moment being at the time at the time of the demand.
If the accused had drank alcohol within 15 minutes of the first test may render the ASD test unreliable. The mere possibility of consuming alcohol within 15 minutes does not affect the reliability of the ASD. The main issue is whether there was any evidence which may have caused the officer to investigate when the accused had his last drink. If "credible evidence" exists the officer should delay test for 15 minutes to ensure reliability.
- R v Loewen, 2010 ABCA 255 (CanLII) at para 18
R v Stauch, 2007 ABQB 85 (CanLII), (2007) AJ No 142 (QB)
R v Carson (2009) OJ No 660
R v Gannon, 2007 ABPC 65 (CanLII)
R v Redstar, 2009 ABPC 79 (CanLII)
R v Tellefson, 2009 ABPC 159 (CanLII)
R v Lindsay, 1999 CanLII 4301 (ON CA),  OJ No 870, 134 CCC (3d) 159
R v Butchko, 2004 SKCA 159 (CanLII),  SJ No 735, 192 CCC (3d) 552
- R v Rasheed, 2009 ONCJ 41 (CanLII) at para 20
- R v Aujla, 2011 ONCJ 10 (CanLII) at para 36
- R v Zoravkovic 1998 CanLII 3202 (ON CA), (1998), 112 O.A.C. 119 (Ont. C.A.)
R v Imanse, 2010 BCSC 446 (CanLII))
R v Dietz,  A.J. No. 45 (C.A.)
R v Church, 2008 BCSC 686 (CanLII)
R v Donald (No.2), 2010 SKPC 123 (CanLII)
- R v Shaw, 2011 SKQB 425 (CanLII)
- R v Einarson, 2004 CanLII 19570 (ON CA), (2004) 183 CCC (3d) at para 35
- R v Szybunka, 2005 ABCA 422 (CanLII) at para 8
"Sniffer dog" searches, most typically seen at border-crossings including airports, can be permitted on the standard of reasonable suspicion.
For a sniffer dog search to be valid, the court must ask itself:
- did the officer subjectively believe that there were reasonable grounds to suspect that the accused was in possession of the drugs?
- were there sufficient grounds to reasonably suspect the accused was in possession of drugs?
Reasonable suspicion in this circumstances requires an "expectation" that the accused is "possibly engaged in some criminal activity. As well, the suspicion must be supported by facts that can be subject to review."
As part of the determination of reasonable suspicion it includes the presence of a "masking agent" such as perfumes, colognes or other odour producing products. 
A observed hand-to-hand exchange in isolation can only amount to a suspicion and nothing more as it may equally be an innocent exchange of small objects.