No critical skill should rest on one person, and training two others to competence gives three holders, each of whom lowers the chance that the whole skill is lost. The arithmetic only holds if the holders fail independently and each can still do the task, so choose people from different households, ages and workplaces, and make them practise.
The rule that critical skills need several trained holders, with the probability arithmetic behind it and the ways it fails.
The Parent’s Rule
The parent article’s rule is: identify the primary holder, train at least two others to competence, record the procedure in writing if possible, and include it in the curriculum. Two others plus the primary holder make three. This article works out why three, and what undermines it. The related article Redundancy: Multiple Carriers makes the independence point for oral items; this one adds the numbers and the competence condition.
Worked Example: Probability Of Losing A Skill
The probabilities below are assumed for illustration. They are not data from any source.
Assume each holder has a 10 per cent chance, which is 0.1, of being unavailable (dead, sick, absent) over a given season, and that holders fail independently.
- One holder: chance the skill is lost is 0.1, or 10 per cent.
- Two holders: both unavailable has chance 0.1 times 0.1, which is 0.01, or 1 per cent.
- Three holders: 0.1 times 0.1 times 0.1, which is 0.001, or 0.1 per cent.
Each added independent holder multiplies the chance of total loss by 0.1 once more. If two of the three live in one household and share the same fire, illness and fate, they behave as one holder, so there are effectively two independent holders and the chance of loss is 0.01, not 0.001. The assumed 10 per cent is only a stand-in: the true chance is different for each community and each skill.
Three Ways The Arithmetic Fails
- Shared fate. Holders in the same house, workplace, tent or age group are lost together by fire, flood, epidemic or conflict.
- Shared error. If all three learned from one teacher, they share that teacher’s mistakes. Teach one of them from a second source where possible, and check procedures against a written reference.
- Lost competence. A holder who has not done the task for years is not a holder. The three must practise or be tested. See Skill Demonstrations and Skill Mastery Recognition.
Method: Building A Real Redundancy
Step 1: Take a red row from the inventory (Critical Knowledge Inventory).
Step 2: Choose the second and third learners from different households and, where possible, different ages, including one young learner who will outlast the others.
Step 3: Train by the sequence in Learning by Doing.
Step 4: Test each by demonstration and write down who passed and when.
Step 5: Set a practice schedule so that each holder performs the task regularly.
Step 6: Record the procedure; see From Memory to Record.
Where To Stop
Training costs a teacher’s time, and children and adults have other jobs. Three trained holders is the parent’s target for each critical skill; for many skills that is sufficient, and the extra teaching time is better spent on the next red row. A skill used daily by many people, such as making a fire, is already widely held.
Common Mistakes
- Counting a spouse and a child as separate holders.
- Training three people on the same day and never again.
- Using the redundancy principle to ignore the written record, which is a separate protection.
What’s Next
- Learn next: From Memory to Record
- Combine with: Recording & Backup Systems