In a hatchery, a break out is often performed to see if we have a problem in our incubation process, or just to establish a standard for a flock, time of the year, machine etc.
There are a lot of break out forms available from different companies, the breeding companies, the incubator manufacturers etc. Although all forms are slightly different, they all come down to the same concept. You take a number of trays, usually three or five, count the viable, first grade chicks, count the deads on the tray and the cull chicks, and open the eggs that did not hatch. You divide the unhatched ones into infertile, early deads, mid deads and late deads, and you classify those categories into sub categories like cracks, contaminated, upside down etc.
And then you calculate the different categories (number of chicks, infertile, early deads, middle deads, late deads etc) as a percentage of the original number of eggs on the trays, and you compare it with the standards for that breed and age. Based on the percentages you can then identify if you have to look into egg storage, upside down setting, egg cracking, temperature settings etc.
This is the widely accepted procedure and it works quite well. However, this method has one down side, and that is the calculation as a percentage of the total eggs. Because if you think about it, an infertile egg can't give a chicken but also can't die. So we are quite familiar with the term "hatch of fertile" next to the term "hatch of eggs set" as it gives an indication of the succes of the hatchery, and not a combined succes of the hatchery and the breeder farm (fertility).
But this is not common in analyzing the mortality during the incubation process. In there we just calculate it as "deads (early, middle or late) of eggs set, ignoring the fact that infertile eggs that do not contain a living embryo can not produce a dead embryo.
Normally this is not a problem, but if the fertility is significantly different from the standard, we might be overlooking a problem as we are not alarmed on it. If we for instance have a tray of 100 eggs and we find 3 early dead embryos, we conclude that we have 3% early deads. That might be a bit higher than we expected, but nothing to really worry about. However, if the fertility on that tray was only 30%, we had only 30 life embryos on that tray that had the possiblity to die, and 3 of them actually did. This means that the real early dead was 10% and not 3%, a percentage that let all the alarmbells go off.
In a normal situation the procedure of "deads of eggs set" is quite sufficient. But if we have an issue with fertility, perhaps because we are working with grand parents and not with parent stock, we need to rethink our judgement. As in a break out we determine fertility anyway, it is a good practice to not only calculate "deads of eggs set" but also "deads of fertile eggs", just as we do "hatch of egg set" and "hatch of fertiles". In this way we can judge the performance of the hatchery in a better, more objective and repeatable way.