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Recognising Situations in context aware systems using Dempster-Shafer Theory Dr. Susan McKeever Nov 4 th 2013.

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Presentation on theme: "Recognising Situations in context aware systems using Dempster-Shafer Theory Dr. Susan McKeever Nov 4 th 2013."— Presentation transcript:

1 Recognising Situations in context aware systems using Dempster-Shafer Theory Dr. Susan McKeever Nov 4 th 2013

2 Context Aware systems – e.g. Smart home Sensors in a smart home Situation tracking – what is the user doing? What activity are they undertaking? E.g Monitoring elderly

3 Context Aware systems Pervasive /ubiquitious /ambient systems – embedded in the environment s E.g. intelligent homes, location tracking system They understand their own “context”. Context-awareness is the ability to track the state of the environment in order to identify situations Situations are human understandable representations of the environment, derived from sensor data

4 Research focus: e.g.Gator Tech Smart home

5 Van Kasteren sensored smart home 14 digital sensors For a month: 7 Situations: Preparing breakfast, dinner, drink, leave house, use toilet, take shower, go to bed

6 Abstracting sensor data to situations Location sensor reading (X,Y,Z, ID239, 12:30:04) Sensor 1, 2, 3 Abstracted Context Situations John located in time 12:30 John is ‘preparing meal’ Is abstracted to Is evidence of Sensor 1, 2, 3 Application e.g. elderly alert system

7 Sensor data Situation Recognition Situation(s) occurring at time, t 12:53 preparing breafast (12:53, 0) (2.15,5.04,3.16, 12:34) Situation Recognition Knowledge Expert? Past data? Situation recognition is a critical, continuous, dynamic process – often required in real time. The recognition process is difficult and uncertain – no single approach suitable for all

8 Situation Recognition - Scenario Scenario “The person is in the kitchen. It is morning time. They carry out a series of tasks, such as taking cereal out of the groceries cupboard, using the kettle, opening the fridge, and using the toaster” Human Observer: “preparing breakfast” Why? Individual tasks may not confirm that breakfast is in progress, but together, indicate the ’preparing breakfast’ situation. Morning time Informative sensors e.g. toaster

9 Recognising situations – Automated Sensor overlap - Kettle and fridge: ’preparing drink? Different people “prepare breakfast” in different ways.. Individual efinitions? Gaps of seconds or minutes occuring with no sensor activity – classify? Sensors can breakdown and have error rate – toaster sensor doesn’t fire? As more tasks are done, system is more certain of ‘preparing breakfast situation’ – Temporal aspect The person does not prepare breakfast in the same way every day. The tasks are not necessarily performed in any particular order. Co-occurring situations? (’on telephone’); Cannot o-occur (’user asleep’)? -Valid combinations of situations. A second occupant now enters the kitchen – how to distinguish?

10 Recognising situations – Some approaches Machine learning techniques, inc. Bayesian networks Decision trees Hidden Markhov models reliant on training data Specification based approaches, inc. Logic approaches Fuzzy logic Temporal logic

11 Problems to be solved ( not exhaustive ) How to recognise situations in pervasive environments, allowing for particular challenges: 1.Uncertainty (sensor data, situation definitions, context fuzziness) 2.Difficulties in obtaining training data My solution: Use and enhance evidence theory (Dempster Shafer theory)

12 Why Dempster Shafer theory Devised in 1970s Mathematical theory for combining separate pieces of information (evidence) to calculate the belief in an event. Applied in military applications, cartography, image processing, expert systems, risk management, robotics and medical diagnosis Key features: (1) its ability to specifically quantify and preserve uncertainty (2)its facility for assigning evidence to combinations Various researchers applying in pervasive systems

13 Approach Apply Dempster Shafer (evidence) theory to situation recognition Create a network structure to propagate evidence from sensors Extend the theory to allow for: New operations needed support evidence processing of situation Temporal features of situation Rich (static and dynamic) sensor quality

14 Dempster Shafer theory: Example Two sensors are used to detect user location in an office. The locations of interest are: (1) Cafe, (2) the user’s desk, (3) the meeting room and (4) ‘lobby’ in the building. Meeting room Café User’s desk Lobby Sensor 1 Sensor 2 Any uncertainty is assigned to ‘ignorance’ hypthesis – {desk ^ cafe ^ meetingRoom ^ lobby} Frame of Discernment ‘hypotheses’ (allows combinations) Each sensors assigns belief as a ‘mass function’ which totals per sensor to 1 Evidence sources

15 Dempster Shafer theory: Example Sensor 1 Detects the user’s location in the cafe. The sensor is 70% reliable, so its belief is assigned across the frame as {cafe 0:7; 0:3 ) Sensor 2 The second sensor has conflicting evidence, assigning {meetingRoom 0:2, desk ^cafe^lobby 0:6, 0:2 } To combine evidence source: Use dempster combination rule mass functions

16 Dempster Shafer theory: Combination rule M 12 (A) is the combination of two evidence sources or mass functions for a hypotheses A. Denominator is a normalisation factor 1-K where K = conflicting evidence Evidence sources must sum to 1:

17 Dempster Shafer theory: example Conflict (K ) = 0.14 ; All evidence is normalised by 1-K giving: Café 0.65; meeting 0.07; desk/café/lobby 0.21, uncertainty 0.07 Sensor 1 Sensor 2 Combined evidence

18 Dempster Shafer theory: problems Zadeh’s paradox Conflicting sensor: Appear to agree completely if any agreement – not intuitive

19 Dempster Shafer theory: problems Single sensor dominance A single sensor can overrule a majority of agreeing sensors if it disagrees: e.G.if 5 sensors determine a user location in a house, a single “categorical” (certain) sensor that assigns all its belief to a contradictory option will negate the evidence from the remaining 4. Sensor 1 Sensor 2Sensor 3 Sensor5 Sensor 4 Kitchen 0.7 Kitchen 0.6 Kitchen 0.8 Kitchen 0.9 Sitting room 1

20 Dempster Shafer theory: gaps No support for evidence spread over time. Assumes evidence is all co-occuring but in reality evidence may be spread over time. e.g. detecting “prepare dinner” situation detected by sensors on cupboards and fridges. Groceries Cupboard Accessed Fridge Accessed Freezer Accessed Pans Cupboard Accessed Plates Cupboard Accessed Prepare Dinner Timeline 40 minutes

21 Dempster Shafer theory: gaps Only deals with fusing evidence: no “theory” for propogating evidence across other rules in order to recognise situations Limited to just combining n “sources”: Need a set of additional mathemtical operations for propogating evidence Sensor 1, 2, 3 Abstracte d Context Situations Sensor 1, 2, 3 Abstracte d Context Situations Sensor 1, 2, 3 Abstracte d Context Situations Sensor 1, 2, 3 Location sensor reading (X,Y,Z, ID239, 12:30:04) John located in time 12:30 John is ‘preparing meal’ Is abstracted to Is evidence of

22 sensor Sensor Context Value situation Situation Sensor Context Value Context Value Context Value Context Value Context Value Context Value Certainty 0.n Certainty 0.n Certainty 0.n Sensor Level Abstracted Context Situations sensor situation Dempster Shafer theory: gaps Only deals with fusing evidence: no “theory” for propogating evidence across other rules in order to recognise situations (and a way to capture all this knowledge)

23 Recognising situations – Using Dempster Shafer theory Want an approach that reduces or eliminates reliance on training data. OK (provided we can define mass functions to say what sensor readings mean) That allows for “uncertainty” OK That allows temporal information to be included To be added That allows sensors belief to be propogated (distributed) up into situation hierachies based on “knowledge” rules To be added That addresses the issue of Zadeh’s paradox and dominant sensors To be added Ultimately: Develop a full decision making architecture for real time situation recognition (overleaf) To be added Needed to extend Dempster Shafer theory

24 Knowlege Sensor Readings Belief Distribution Decision Stage Recognised Situations Valid situation combinations At time t Applicati -ons Develop a full decision making architecture for real time situation recognition using extended DS theory Extended DS theory Prep Breakfast 0.3, Take a shower 0.6

25 Knowledge: an interconnected hierarchy of sensor and situations sensor Sensor Context Value situation Situation Sensor Context Value Context Value Context Value Context Value Context Value Context Value Certainty 0.n Certainty 0.n Certainty 0.n Sensor Level Abstracted Context Situations sensor situation

26 Plates Used Cup Used Fridge Used Groceries Used Microwave Used Pans Used Freezer Used Get Drink Prepare Breakfast Prepare Dinner Morning Plates Cupboard Cup Fridge Groceries Cupboard MicrowavePans Cupboadr FreezerTime Moning Nighttime VanKasteren e.g. 3 of the situations

27 First : Define a notation for knowledge capture : denoting sensor evidence /context/ situations – Situation DAG sensor Sensor Situation Context Value Certainty 0.n Certainty 0.n Certainty 0.n Discount 0.n > 10 > Context Value Context Value Context Value Context Value Context Value Belief distribution Situations Sensors Context Values Belief distribution

28 First : Define a notation for denoting sensor evidence /context/ situations – Situation DAG i.e to capture the knowledge of what sensors indicate what situation is a type of is evidence of Duration of situation, evidence not in sequence Duration of situation, evidence in sequence >duration > Sensor, context value or situation Discount 0.n Discount factor applied to a sensor: 0< n <1 Certainty 0.nCertainty applied to an inference rule: 0 < n < 1

29 Second: Create evidence propogation rules to distribute/propogate belief up to situation level sensor Sensor Context Value situation Situation Sensor Context Value Context Value Context Value Context Value Context Value Context Value Certainty 0.n Certainty 0.n Certainty 0.n Sensor Level Abstracted Context Situations sensor situation Translate Sensor readings into beliefs here.. Up to situation certainties here

30 Second: Create evidence propogation rules to distribute/propogate belief up to situation level sensor Sensor Context Value situation Situation Sensor Context Value Context Value Context Value Context Value Context Value Context Value Certainty 0.n Certainty 0.n Certainty 0.n Sensor Level Abstracted Context Situations sensor situation

31 Is a type of: e.g. Situation X is occuring if either Situation Y OR Z is occuring Occupant is “resting” if they are “watching TV” or “in bed” Second: Create evidence propogation rules to distribute/propogate belief up to situation level: Examples Distributing combined belief across single situations

32 Second: Create evidence propogation rules to distribute/propogate belief up to situation level: Examples: Sensor Quality Some sensors are inherently lower quality as an evidence source e.g. Calendar sensor is indicative of real calendar owner’s location 70% of the time – Discount (d) evidence from the sensor

33 Third: Include temporal evidence: Groceries Cupboard Accessed Grocery Cupboard accessed Freezer Accessed Plates Cupboard Accessed Fridge Accessed Prepare Dinner Timeline 40 minutes Different Sensors fire intermittently – no single sensor sufficient for situation recognition (1) Use absolute time as evidence (2) Find a way to combine transitory evidence

34 Groceries Cupboard Accessed Fridge Accessed Freezer Accessed Pans Cupboard Accessed Plates Cupboard Accessed Prepare Dinner: Time Extended Evidence Time Fridge Extended Fridge Extended Fridge Extended Fridge Extended Fridge Extended Groceries Cupboard Extended Groceries Cupboard Extended Groceries Cupboard Extended Groceries Cupboard Extended Plates Cupboard Extended Plates Cupboard Extended Plates Cupboard Extended Freezer Extended Freezer Extended Pans Cupboard Extended Prepare Dinner Starts Prepare Breakfast Ends Situation Duration Third: extend evidence for duration of situation

35 Fusing time extended evidence: Adjust Dempster Shafer fusion rules to allow for time extension of evidence Two transitory extended mass functions for hypothesis h with duration t dur, a t time t +t rem

36 Fourth: Allow for Zadeh’s and Single sensor dominance Use an alternative combination rule (Murphy’s) which averages out the evidence BEFORE fusing Use a simpler averaging rule to fuse evidence Lacks convergence Removes Zadeh’s problem Two options:

37 Fifth: Combine all this and apply to real world data for situation recogntion Knowlege Sensor Readings Belief Distribution Decisio n Stage Recognised Situations Valid situation combinations At time t Applica ti-ons Extended DS theory Prep Breakfast 0.3, Take a shower 0.6 Test our approach using annotated datasets of sensor readings

38 Experiments Data set (1) “Van Kasteren” Heavily used by other researchers - compare results on situation recognition 7 situation annotated, 14 sensors Data set (2) “CASL” Office data set: 3 situations annotated, Location sensors, Calendar sensor, Keyboard sensor

39 QuestionData set 1How accuracy is our DS approach for situation recognition? Both 2Do DS temporal extensions improve situation recognition? Van Kasteren 3Do DS quality extensions improve situation recognition? CASL Evaluation Various sub questions also addressed: comparison with published results, comparison of DS fusion rules, impact of quality on situation transitions, quality parameter sensitivity, static versus dynamic quality

40 Evaluation 1.2 annotated published real world datasets – VanKasteren (Smart home) and CASL (office-based) 2.Situation DAGs created for both datasets 3.Situation recognition accuracy measured using f- measure of timesliced data sets; 4.Recognition accuracy using temporal and quality extensions evaluated 5.J45 Decision Tree and Naive Bayes used for comparison, and published results ; Cross validation used.

41 Use of DS theory with temporal extensions for situation recognition F-Measure for each situation using DS theory – (1) no time, (2) absolute time, (3) time extended (VanKasteren dataset )

42 Temporal DS theory compared to two other approches: Naïve Bayes, J48 decision tree. Situations

43 Our approach compared to the three available published results Same experimental measures * Excludes timeslices with no sensors firing which are harder to infer – ‘inactive’ Timeslices harder to infer *

44 Use of DS theory with temporal extensions Use of temporal extensions significantly improves situation accuracy (over baseline DS theory alone) Performs better than J45, Naive Bayes (particularly with limited training data). This improvement narrows when more training data used (LODO) Achieves 69% class accuracy in comparison to VanKasteren (49.2%) and Ye*(88.3%)

45 Use of DS theory with quality extensions F-Measure for each situation using DS theory – with and without quality

46 Use of quality parameters significantly improves situation recognition accuracy (over baseline) Performance close to Naive Bayes (4%) and J48 (2%) - Each individual sensor’s quality contributes to improvement Sensitivity analysis of quality parameters indicates the relative quality of sensors may be important Time based dynamic quality parameters impact situation transitions – application dependant Use of DS theory with quality extensions

47 Our DS theory is a viable approach to situation recognition: Not reliant on training data Incorporates domain knowledge Caters for uncertainty Encoding temporal and quality knowledge improves performance over basic DS approach BUT Knowledge must be available Different fusion rules appropriate in different scenarios – requires expert “evidence theory” knowledge Environment changes – no feedback loop for drift Potentially high computation effort can be reduced Conclusions

48 Contributions 1.A situation recognition approach based on DS theory 2.Selection of existing and creation of new evidential operations and algorithms to create evidence decision networks 3.Temporal and quality extensions to DS theory 4.Diagramming technique to capture structure of evidence for an environment (Situation DAG) 5.A thorough application, evaluation and analysis of the extended DS theory approach 6.An analysis of alternative fusion rules

49 Related Publications Journal 1.Journal of Pervasive and Mobile Computing 2.JAISE Volume 2, Number International Conferences 1.EuroSSC Smart Sensing UK ICITST Pervasive Services Italy 2008 International (Peer viewed) Workshops 1.Pervasive 2010, Helsinki, Finland 2.CHI 2009 Boston, US 3.QualConn 2009, Stuttgart, Germany 4.Pervasive 2009, Sydney, Australia,

50 Questions?

51 Experiments Establish situation DAG for each dataset System Developers -Users -Application experts Sensors Context Values Situations


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