As long as you cite the previous work, it is probably fine to make the comparison, as this policy is precisely what meta analytic approaches have to do: compile previous results and consider differences among them.

The shortcoming that reviewers and editors would be concerned about is the potential disparity in measurement circumstances between the previous work and your work.  The reviewers will be curious how you will guarantee that the comparison is not going to be biased by the different conditions and how similar you can show these conditions to have been.  One good first step may be to contact the author of the earlier work and see whether the author is amenable to sharing the data. That way, you could go into the contrast knowing as much as possible.

Best, Damian

As long as you cite the previous work, it is probably fine to make the comparison, as this policy is precisely what meta analytic approaches have to do: compile previous results and consider differences among them.

The shortcoming that reviewers and editors would be concerned about is the potential disparity in measurement circumstances between the previous work and your work.  The reviewers will be curious how you will guarantee that the comparison is not going to be biased by the different conditions and how similar you can show these conditions to have been.  One good first step may be to contact the author of the earlier work and see whether the author is amenable to sharing the data. That way, you could go into the contrast knowing as much as possible.

Best, Damian

Many thanks to you, what I meant here is for example I got total mean of 5.3 log cfu/eggshell, other researcher in 2009 got 3.5 cfu/eggshell, and other one got 2.5 log cfu/eggshell in 2002, in this case can I compare my result with those researcher results to see if the ban of washing eggs after 2012 has an effect on increasing the total microbial load. My hypothesis is the total microbial load has tendency to increase since the eggs are not washed anymore. 

Many thanks to you Paul what type of analytical test would you suggest, for example if I assume that washing eggs and none washing eggs are my independant variables, and  am going to run ANOVA test to compare 3 dependant means of total microbial load on 95% confidence intervals to test the significance between the three variables. Is this the right approach? 

Egg washing has not been allowed in Europe (EU) for decades already!.

The obligatory change from battery housing to floor housing in the EU, might be one important factor if CFU numbers of table eggs in consumer channels have really gone up.

That is another important factor but I think the microbial load would decrease since the hens were more stressed and prone to microbial attack in battery cages than in barn. 

My (educated) guess would be that external factors such as microbial load of the laying nests used by many hens during the day are more important for the microbial load of the egg shell, than the internal ones i.e. microbes present in the oviduct during egg shell formation.

I wouldn't put these three results in a graph going into a publication or conference presentation. For a class/lab presentation, this reference will help people visualize and remember and maybe start discussing afterwards. In a paper, I would just cite the other references in the discussion section, for example. My advice, as suggested above, is to run the experiments with unwashed eggs yourself.

Now, if you were doing a meta-analysis and looking at all the research on unwashed eggs and bacteria load, it would be acceptable to compare results; even so, you would need to know the different experimental conditions that explain the variation in results. But comparing washed and unwashed eggs is a bit like comparing apples and oranges in my mind and a lot of factors could confound the results.

So, yes, you could present it as a worthy hypothesis to explore and then actually test it yourself.

The answer must be yes, otherwise I see little point in the scientific literature. If my results only apply to one grower 2.5 kilometers east of Paris, then so what? The hope is that my results from that one study have broader application. Your task is to determine how broad that application should be. Are the other studies and your research broad enough that the problems people have mentioned are not important? If you measured several hundred eggs from several dozen producers in several countries and the other studies did the same, then your case is stronger. If all the studies consist of measuring 10 eggs from one henhouse (different henhouse for each study), then there are insurmountable problems. I would guess that the real situation is something between these extremes.

What bothers me most in the posted comments is that Kees' stated that egg washing has been banned for decades. If that is true, then Mohammed's cited articles from 2002 and 2009 would not have washed the eggs, and the hypothesis is not valid. It will be very important to establish dates for laws that forced washing, and when the laws were changed to ban washing. If the former do not exist, then I suspect there would be a large number of producers who wouldn't wash the eggs even before the law to ban washing. It would then be critical to read the methods sections and figure out if the eggs in those studies were washed or unwashed.

Timothy: according to my recollection egg washing was banned in the EU somewhere in the sixties. When I entered poultry research in the Netherlands in1971 some collegues told me about the experiments they had done in that time about egg washing.

On the other hand egg washing, oiling and cooling was quite common in the US. These eggs could be stored for weeks (and thus transported all over the world) and thus be a firm competitor to european poultry farmers that were not allowed to wash, oil and cool the eggs meant for table consumption.

Kees: Thank you. With that response I searched the literature to see what a keyword search containing "Colony forming units eggs" would turn up. I quickly found a study out of Poland:

Bacterial pollution of consumer egg surfaces with regards to the maintenance system of laying hens (2014) Belkot, Zbigniew, and Gondek, Michal. MEDYCYNA WETERYNARYJNA-VETERINARY MEDICINE-SCIENCE AND PRACTICE, Volume:70(6): 378-382.

They measured different types of bacteria on eggs from hens in different environments. The log values ranged from 0.65 to 5.35 depending on the type of bacteria and the treatment of the hens.

Because it was so easy to find more data and because Mohammad's hypothesis appears flawed, I think the first step is to do a more careful literature review. From Kees' response, I suspect there will be considerable literature that predates electronic bibliographies. Also, the degree of variability reported in the Polish paper for most bacterial classes was about 1 order of magnitude, but for the Enterococcus it was 2 orders of magnitude and for the Staphylococcus it was 7 orders of magnitude (assuming that the shells from caged hens really had a log value of -2.73 and this wasn't a misprint or misunderstanding on my part due to only reading the abstract).

As Damian said, the analysis of multiple sources of data is a process called a meta-analysis. The concept is quite useful, but this application of that idea seems to require more refining before taking that step.

One other remark; my statement on the ban of egg washing relates to table eggs for human consumption.

There are two other (large) categories of eggs on the market as well: those to be used by the egg products industry and hatching eggs.

This last category goes to conditions ideal for microbial growth if sufficient nutrients should be available.

Some studies might have been carried out on one of these groups. This might influence results as well.

Thanks for your explaining, since there are more variables would bias the comparison, and each experiment went through a different treatment, I would guess stating that in the discussion part that the washing ban might have an influence on total microbial load according to the previous study might be enough.