Hierarchical Task analysisDr. Yan LiuDepartment of Biomedical, Industrial and Human Factors EngineeringWright State University2Introduction What is Task Analysis The process of analyzing the way people perform their tasks  The things they do The things they act on The things they need to knowTo clean the house• Get the vacuum cleaner out• Fix the appropriate attachments• Clean the rooms• When the dust bag gets full, empty it• Put the vacuum cleaner and tools awayMust know about:vacuum cleaners, their attachments, dust bags, cupboards, rooms, etc.3Approaches to Task Analysis Task Decomposition A top-down process in which a task is split into subtasks by sequence Cognitive Task Analysis The extension of traditional task analysis techniques to yield information about the knowledge, thought processes and goal structures that underlie observable task performance4Task Decomposition Aims Describe the actions people do Structure them within a task-subtask hierarchy Describe order of subtasks Hierarchical Task Analysis (HTA) Outputs are a hierarchy of tasks and subtasks and plans describing in what order and under what conditions subtasks are performed Shown as textual descriptions or diagrams Information may be more accessible at a glance with diagrams, especially in hierarchies with many levels50. in order to clean the house1. get the vacuum cleaner out2. get the appropriate attachment3. clean the rooms3.1. clean the hall3.2. clean the living rooms3.3. clean the bedrooms4. empty the dust bag5. put vacuum cleaner and attachments awayPlansPlan 0: do 1 - 2 - 3 - 5 in that order. when the dust bag gets full do 4Plan 3: do any of 3.1, 3.2 or 3.3 in any order depending on which rooms need cleaningTextual HTA of the Task of Cleaning a House• Indentation is used to denote the levels in the task hierarchy • Plans are labeled by the tasks they correspond to • Only the plans denote the order of task performance• Not all subtasks need to performed, and not necessarily in the order presented in the hierarchy60. Clean the house1. Get the vacuum cleaner out2. Fix the appropriate attachment3. Clean the rooms4. Empty the dust bag5. Put vacuum cleaner and attachments awayPlan 0: do 1 - 2 - 3 - 5 in that order. when the dust bag gets full do 43.1. Clean the hall3.2 Clean the living room3.3. Clean the bedroomsPlan 3: do any of 3.1, 3.2 or 3.3 in any order depending on which rooms need cleaningDiagrammatic HTA of the Task of Cleaning a House7Generating Hierarchy Procedure Identify the task to be analyzed  Break the task down into subtasks  What subtasks must be accomplished in order to perform the main task Refer to various sources  e.g. direct observation, expert opinion, documentation, etc. Decide upon the level of detail into which to further decompose the subtasks (with some stopping rule) Continue the decomposition process, ensuring that the decompositions and numbering are consistent Group some subtasks (if too detailed) at a certain level into higher-level subtasks Present the hierarchy to a domain expert to check for errors or omissions8Stopping Rule Depends on the Purpose of Task Analysis Put more effort into those subtasks which are directly relevant to the intended purpose0. In an emergency situation in a chemical plant1. read the alarms2. work out appropriate corrective action3. perform corrective actionIf our ultimate aim is to install computer monitoring of the plant, then we would be interested in expanding subtasks 1 and 3. If the aim is to produce online operations manuals, then subtask 2 would require expansion9Stopping Rule (Cont’d) P × C rule Particularly appropriate when the aim is to design training materials If the probability of making a mistake in task (P) multiplied by the cost of the mistake (C) is below a threshold, then stop expanding Simple tasks need not to be expanded unless they are critical Stop at the point where the task contains purely muscle actions or involves purely cognitive activities e.g. mouse movement, recall someone’s name100. make a cup of tea1. Boil water2. Empty pot3. Put tea leaves in pot4. Pour in boiling water5. Wait for 4 or 5 minutesPlan 0: do 1 At the same time, if the pot is full, do 2Then do 3 – 4 After four or five minutes do 61.1. Fill kettle1.2 Put kettle on stove1.3. Wait for kettle to boil6. Pour teaPlan 1: do 1.1 – 1.2 – 1.3 when kettle boils, do 1.41.4. Turn off gasAny omission or error? Can some first-level subtasks be combined ? HTA of the Task of Making a Cup of Tea111. Boil water2. Empty pot3. Make pot of tea3.2. Pour in boiling water4. Wait for 4 or 5 minutes1.1. Fill kettle1.2 Put kettle on stove1.4. Wait for kettle to boil5. Pour teaPlan 1: do 1.1 – 1.2 – 1.3 – 1.4when kettle boils, do 1.51.5. Turn off gas3.1. Warm pot3.2. Put tea leaves in potPlan 3: do 3.1 – 3.2 – 3.3 1.3. Turn on gasCan we expand 5?0. make a cup of teaPlan 0: do 1 At the same time, if the pot is full, do 2Then do 3 – 4 After four or five minutes do 512Suppose subtask 5 “Pour tea” can be further decomposed5. Pour tea5.1. put milk in cup5.2. fill cup with tea5.3. add sugar to tastePlan 5.Do 5.1 – 5.2 – 5.3What if we want to make more than one cup?5.15.2More cup(s)?Yes5.3130. make cups of tea1. Boil water2. Empty pot3. Make pot3.2. Pour in boiling water4. Wait for 4 or 5 minutesPlan 0: do 1 At the same time, if the pot is full, do 2Then do 3 – 4 After 4 or 5 minutes do 61.1. Fill kettle1.2 Put kettle on stove1.4. Wait for kettle to boil5. Pour teaPlan 1: do 1.1 – 1.2 – 1.3 – 1.4when kettle boils, do 1.51.5. Turn off gas3.1. Warm pot3.2. Put tea leaves in potPlan 3: do 3.1 – 3.2 – 3.3 1.3. Turn on gas5.1. Put milk in cup5.2. Fill up with tea5.3. Add sugarPlan 5: 5.15.2More cup(s)?Yes5.314Waiting Represent Waiting  Considered as a subtask if it is a “busy” waiting The person may be chatting while the tea brews Included in the plan if time seems critical for the task sequence  Wait for 4 or 5 minutes Perhaps a little redundant in this example, but task analysis is not an exact science redundant?Plan or subtask?15Types of Plan Fixed Sequence The same sequence of subtasks is always followed Plan 3 in the HTA of tea making Optional Subtasks Subtasks that may or may not be performed depending on circumstances Subtask 2 in plan 1 in the HTA of tea making  Waiting-for