過去的十年中，國際養豬生產發生了很大變化，包括更多地採用全進全出、多點式和按日齡分群的生產系統，以及在飼養育肥豬時更多採用隔間，從而減少病原體傳播。這些變化大多更容易在大型豬群中實現，因此，豬群規模大與疾病風險之間的正相關關係也許不復存在。
In the last decade there have been substantial changes in swine production internationally, including the increased implementation of all-in, all-out, multisite and age-segregated production systems, and greater use of compartments for finishing pigs to reduce pathogen transmission. Many of these changes can be implemented more readily in large herds, and hence the historic positive relationships between larger herd size and disease risk may no longer exist.
雖然已發表研究和數據集的再分析大多是關於丹麥的養豬生產，但涉及的基本問題卻廣泛適用於其他國家及生產系統。因此，針對豬群規模、豬群密度、豬的飼養密度以及其他管理和環境因素，有必要採取更科學的方法來確定其對疾病風險造成的影響。對於疾病控制而言，尤其要注意確定豬群規模效應的真偽，以便給出適當的緩解建議。
Although many of the published studies and data sets that we reanalysed involved Danish swine production, the underlying issues are widely applicable to other countries and production systems. Accordingly, we believe that more structured approaches are necessary to determine how herd size, herd density, pig stocking density and other management and environmental factors affect disease risk. For disease control decisions, it is especially important to determine whether any herd-size effect is real or spurious so that appropriate advice about mitigations can be given.
為確保今後關於豬群規模與疾病之間關係的研究成果達到最大效用，現提出如下建議。
To ensure the greatest utility of findings from future studies on the relationship between herd size and disease, we make the following recommendations.
（i）在對豬群規模進行度量時，最好是能站在疾病或生產利益的角度闡述群體所處的風險。對於病原而言，理論上是指易感豬只數量，但實際上，可能僅是每棟建築物或隔間內豬只數量或總數。特別是對斷奶豬和育成/育肥豬的疾病，建議測量豬只的密度（就所佔面積和空間而言）。針對各類規模度量的複合評估或單項評估，應包含於豬群規模相關的分析結果中。飼養密度及空間的標準已得到廣泛認可，相比其他豬群規模的度量方法，其相對穩定且分類更加固定，強烈建議將此類評估應用於所有傳染病研究中。此外，改進對豬群規模的描述，有利於剔除諸如「大」這樣的隨意豬群分類，以便可以更好地比較不同國家間的研究成果。
(i) Herd size should be measured in ways that best characterize the population at risk for the disease or productivity measure of interest. For infectious agents, this would ideally be the number of susceptible pigs, but in practice only the total number and number per building or compartment will probably be available. In addition, we recommend measurement of pig density (in terms of floor and air space occupied) especially for diseases of weaned and grower/ finisher pigs. Results of analyses of the herd-size associations should include evaluation of the joint and individual contributions of each type of herd size measure. Because standards for stocking density and air space are well accepted, relatively stable and less subject to arbitrary categorization than other herd size measures, we strongly recommend their evaluation in all studies of infectious disease. Also, improved characterization of herd size will help remove the rather arbitrary classification of herds as 'large' and allow better comparison of results among studies in different countries.
（ii）在豬群、建築物或隔間層面評估管理相關的風險因素時，若無法證明豬群規模是否為相關因素，應將其視為潛在的混雜因素。如果用豬群規模代表其中單個或多個無法衡量的管理風險因素，則尤其要注意調整豬群規模後殘留混雜因素可能依舊存在，除非豬群規模與這些變量之間具有極大相關性（格陵蘭島和紐特拉島，1980年），但此類情況在養豬生產中極不可能發生。此外，由於同一地區豬群規模與豬群（豬只）密度通常相關，因此應弄清兩者之間的關係。
(ii) Studies that evaluate management-related risk factors at the level of herd, building or compartment should consider herd size as a potential confounder, unless there is adequate prior evidence to suggest that herd size is not a causal factor. If herd size is used as a surrogate for one or more unmeasured management risk factors, it is important to be aware that residual confounding may still exist after adjustment for herd size, unless herd size is a perfect correlate for these variables (Greenland and Neutra, 1980), a situation that we believe is most unlikely in swine production systems. In addition, the relationship between herd size and herd (and pig) density in an area should be clarified since these two variables are often associated.
（iii）根據豬只數量研究管理因素之間的內在聯繫，需對豬群規模、豬群密度和豬只密度進行研究。雖然所選統計模型的形式與基本生物學過程之間關係複雜，但多元統計技術的有效性可以更好地定義這些關係（Greenland，1989）。最近，Dohoo等人（1997）對流行病學研究中用於處理大量自變量的統計技術作出文獻綜述。
(iii) Population-based studies of the interrelationships among management factors and between management factors, herd size, herd density and pig density are needed. The availability of multivariable statistical techniques provides a powerful tool to better define these relationships, although no simple relationship exists between forms of the selected statistical models and the underlying biological processes (Greenland, 1989). Statistical techniques to deal with a large number of independent variables in epidemiological studies have been reviewed recently by Dohoo et al. (1997).
（iv）應假設出所有與豬群規模相關的生物學原因。此外，應該充分討論研究潛在的偏差和限制，這些偏差和限制會影響豬群規模與疾病風險之間的正相關性或負相關性或無相關性的因果解釋(Savitz, 1992)。這樣有助於改進研究設計，從而對成功幹預的可能性進行更為嚴格的評估。
(iv) Likely biological reasons for any herd size association should be postulated. Moreover, there should be adequate discussion of the potential biases and limitations of the study that affect the causal interpretation of a positive or negative association or no association between herd size and disease risk (Savitz, 1992). Such candor should also facilitate improvements in study design and lead to a more critical assessment of the likelihood of successful interventions.
（v）當所研究豬群的規模分布與源種群不同時，至少是應提及所研究國家或地區豬群規模的中值(理想情況下應提及豬群規模分布)。風險計算偏離豬群規模中值(連續變量)或將豬群規模中值作為參照組，此應作為闡述豬群規模相關性的標準方法。此方法能夠解釋豬群規模在不同時期出現的改變，以及小於最低規模時，豬群有可能消失的原因。
(v) Whenever the herd size distribution of study herds differs from that of the source population, authors should report at least the median herd size (and ideally the distribution of herd size) for the country or region of the study. For risk calculations deviations from the median herd size (continuous variable) or the use of the category with the population median herd size as the reference group should be used as a standard way to report herd size associations. Such an approach will account for shifts in herd size distribution with time and the likely disappearance of herds smaller than a certain minimum herd size.