It&#x27;s easy to see this in terms of statistics if it&#x27;s not _your_ 2-year old with the speech delay. Not all individuals tolerate lead at the same level. Lead was commonly used in the middle ages and not everyone suffered from the neurological damage that lead can cause.<p>Statistics are the best tool we have but it&#x27;s also important to realize that when the incidence rate is low enough, you need a lot of data for statistical methods to be reliable. We see this in A&#x2F;B testing of conversion rates. If your base conversion rate is only 0.1% and you are looking for a 15% lift (e.g. 15% higher chance of lead poisoning), then you need a sample size of over 700,000 for each branch (i.e. 700,000 kids &quot;exposed&quot; to lead and 700,000 not exposed). That kind of sample size is hard to come by.