Critical Activities
There are 13 critical activities. These are ‘get ingredients’, ‘put water in the pan’, ‘cook pasta shells’, ‘drain pasta’, ‘place pasta on dish’, ‘spoon cream on pasta’, ‘place dish in oven’, ‘remove dish from oven’, ‘serve dish’, ‘eat meal’, ‘cook apple pie’, ‘serve apple pie’ and ‘eat and clean away apple pie’.
The critical activities are activities that cannot be delayed in order for the network to finish on time (at 79 minutes). For example, a delay in ‘cook pasta shells’ will delay the ‘drain pasta’ as it must start at 14.5 minutes. This will have a knock-on effect on the rest of the critical path. The last activity, ‘eat and clean away’ will be delayed at the same amount of time as the original activity ‘cook pasta shells’ was delayed.
Critical activities can be established when:
earliest finishing time (lj) - the latest starting time (ei) = the duration of the activity.
Independent and Interfering Activities
The five independent activities are ‘preheat oven’ ‘get pan’, ‘boil water in kettle’, ‘put layer of tomato sauce on dish’ and ‘prepare boiled sweet corn’. These activities have spare time known as floats. This means they can be delayed for however long their float is without delaying the entire project duration. Floats occur when:
earliest finishing time (lj) - the latest starting time (ei) > the duration of the activity.
For example, ‘boil water in kettle’ starting from 0 minutes takes 2 minutes to finish. Its latest finishing time is 5 minutes so can be delayed by upto 3 minutes. If ‘boil water in kettle’ finishes after five minutes have passed then the next activity ‘put water in pan’ will be delayed too. However, this is a critical activity and as mentioned before it does not have any spare time. As a consequence, if this happends in real life the whole project will be delayed.
The three interfering activities are ‘place broccoli & herbs in processor’, ‘season with Tabasco & salt’ and ‘process to a cream’.
TABLE OF INFORMATION FOR CASCADE CHARTS
A cascade chart represents the activities in a different format. These are bars drawn against time (minutes). This way, it is much easier to see what activities are occurring at any time. Critical activities are shaded in black. Dashed lines indicate which activities follow each other and the latest finishing times and earliest starting times for every activity. So, activity ‘boil water in kettle’ has a dashed line up to 5 minutes. This means this activity must not pass this. This will be taken advantage of later when making a rescheduled cascade chart.
From the cascade chart I can derive how many workers there are during the project in the resource histogram. The number of workers illustrates how many floats are taking place simultaneously during the project. In other words, this also indicates how many people (workers) are required if each were to carry out an activity. For example from 5.5-6 minutes, TWO cooks will be required one to ‘place broccoli and herbs in processor’ and the other to ‘put layer of tomato sauce on dish’ activities.
Activities that do not require a worker throughout their duration are not taken into consideration in the resource histogram. For example, ‘preheat oven’ requires a person to switch on the oven that will take simply a second and is therefore regarded negligible. I have placed a red line along the time axis in the resource histograms where such activities are taking place.
The histogram shows the maximum number of workers of 3 during 5-5 ½ minutes and a minimum number of 0 at various intervals.
Rescheduled Cascade Chart
As can be seen I was able to reduce the number of workers from my resource histogram. I decided not to move ‘get pan’ in the first ½ minute because no matter where I moved it there would still be two workers somewhere between 0-5 minutes. I decided in realistic terms that it would make sense to leave it at the beginning so somebody could leave after thirty seconds as for the rest of the project only one person/worker is required. I moved ‘place broccoli & herbs in processor’ by ½ a minute. This shifted the preceding activities ‘season with Tabasco & salt’ and ‘process to a cream’ by ½ a minute too. In the same interval (5-6 minutes) I moved ‘put layer of tomato sauce on dish’ along by 4 ½ minutes and ‘prepare boiled sweet corn with chillies’ from 5 to 17.5 minutes. This resulted in only 1 worker being required where previously 4 and 3 workers were required. Obviously this meant that at places where before no workers were required, 1 worker was needed i.e from 17.5 to 25.5 (compare resource and resource levelled histograms) to balance.
If I followed this in real life instead of starting the placing the broccoli & herbs in the processor and putting the layer of tomato sauce on the dish at the same time as the network shows, the latter will take place 4 ½ minutes later. The succeeding activity after putting the layer of tomato sauce on the dish is placing pasta on the dish will still be on time.
I feel I have made maximum use of the one worker from after ½ a minute till the end.
‘WHAT IF’ scenarios
When any critical activity on the critical path is delayed the entire duration of the project is delayed to. These have no spare time and therefore no floats. Take critical activity ‘cook apple pie’ for instance, if this is delayed the next activity which is critical too ‘serve apple pie’ will have its starting time delayed too. This must start at 51.5 minutes and end at 61.5minutes. Note that on the cascade charts all critical activities dashed lines show no space to move as they connect straight to the next activity.
When independent or interfering activities are delayed by their float duration they will not affect the rest of the project. In independent activities the float of an activity only relates to that particular activity. For example, when preparing boiled sweet corn with chillies it has 40.5 minutes to spare if it begins on time. This cannot be shared with its receding or succeeding activities, as they are critical. Instead, we can use this extra time to finish putting water in a pan, cooking the pasta and draining the pasta. After this we can then boil the sweet corn with chillies, as there has only been a delay of 10.5 minutes, still leaving enough float.
The three interfering floats precede each other. They share a spare time of 7.5 minutes. This float unlike the independent float can be shared at anyhow among the three. So, for example, I could spend this float by 2 ½ minutes extra in placing the broccoli and herbs in the processor, 2 ½ minutes extra in seasoning with Tabasco and salt and 2 ½ minutes extra in processing to a cream. As an alternative I could spend this 7 ½ minutes float on just one of these activities like processing to a cream. When cooking I would then have to ensure I do not take extra duration in placing the broccoli and herbs in the processor and seasoning with Tabasco and salt.
