FINAL DRAFT UGANDA 
STANDARD  
  FDUS EAS 753
First Edition
2011-mm-dd
 
 Reference numbe r
FDUS EAS 753: 2011
© UNBS 2011
  
Seed potato — Specification 
 FDUS EAS 753: 2011 
i i                 ©  U N B S  2011 - All rights reserved 
 Compliance with this standard does not, of itself confer immunity from legal obligations 
A Uganda Standard does not purport to include all necessary provisions of a contract. Users are 
responsible for its correct application  
© UNBS 2011 
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilised in any 
form or by any means, electronic or mechanical, in cluding photocopying and microfilm, without prior written 
permission from UNBS. 
Requests for permission to reproduce this document should be addressed to 
The Executive Director 
Uganda National Bureau of Standards 
P.O. Box 6329 
Kampala  
Uganda 
Tel: 256 41 505 995 
Fax: 256 41 286 123 
E-mail: unbs@infocom.co.ug  
Web: www.unbs.go.ug  
 FDUS EAS 753: 2011 
© UNBS  2011 - All rights rese rved                iii National foreword 
Uganda National Bureau of Standards (UNBS) is a parastatal under the Ministry  of Tourism, Trade and 
Industry established under Cap 327, of the Laws of U ganda. UNBS is mandated to co-ordinate the elaboration 
of standards and is  
(a) a member of International Organisation for Standardisation (ISO) and  
(b) a contact point for the WHO/FAO Codex A limentarius Commission on Food Standards, and  
(c) the National Enquiry Point on TBT/SPS Agr eements of the World Trade Organisation (WTO). 
The work of preparing Uganda Standards is carried out through Technical Committees. A Technical 
Committee is established to deliberate on standards in a gi ven field or area and consists of representatives of 
consumers, traders, academician s, manufacturers, governme nt and other stakeholders. 
Draft Uganda Standards adopted by the Technical Committ ee are widely circulated to stakeholders and the 
general public for comments. The committee reviews the comments before recommending the draft standards 
for approval and declaration as Uganda Standards by the National Standards Council. 
This Final Draft Uganda Standard, FDUS EAS 753: 2011 , Seed potato  — Specification, is identical with  and 
has been reproduced from an East African Standard, EAS 753: 2011 , Seed potato  — Specification, and is 
being proposed for adoption as a Uganda Standard.  
This standard was developed by the Food and agricu lture Standards Technical Committee (UNBS/TC 2).  
Wherever the words, “East Africa Standard" appear, they should be replaced by "Uganda Standard." EAS 753:2011 
 
 
© EAS 2011                First Edition 2011 
EAST AFRICAN STANDARD 
Seed potato— Specification 
EAST AFRICAN COMMUNITY 
 EAS 753:2011 9 
 
© EAC 2011 – All rights reserved               ii 
 Foreword 
Development of the East African Standards has been necessitated by the need for harmonizing requirements 
governing quality of products and services in Ea st African Community. It is envisaged that through 
harmonized standardization, trade barriers which are encountered when goods and services are exchanged 
within the Community will be removed. 
 
In order to meet the above objectives, the EAC Partner  States have enacted an East African Standardization, 
Quality Assurance, Metrology and Testing Act, 2006 (EAC  SQMT Act, 2006) to make provisions for ensuring 
standardization, quality assurance, metrology and testing of  products produced or originating in a third country 
and traded in the Community in order to facilitate industrial development and trade as well as helping to protect the health and safety of societ y and the environment in the Community. 
 East African Standards are formulated  in accordance with the procedures established by the East African 
Standards Committee. The East African Standards Committe e is established under the provisions of Article 4 
of the EAC SQMT Act, 2006. The Committee is compos ed of representatives of the National Standards 
Bodies in Partner States, together with the repr esentatives from the priv ate sectors and consumer 
organizations. Draft East African Standards are circul ated to stakeholders through the National Standards 
Bodies in the Partner States. The comments received are discussed and incorporated before finalization of 
standards, in accordance with t he procedures of the Community.  
 Article 15(1) of the EAC SQMT Act, 2006 provides that “Wit hin six months of the declar ation of an East African 
Standard, the Partner States shall adopt, without deviation from the approved text of the standard, the East 
African Standard as a national standard and withdraw any existing national standard with similar scope and 
purpose”. 
 
East African Standards are subject to review, to keep pace with technological advances. Users of the East 
African Standards are therefor e expected to ensure that they always hav e the latest versions of the standards 
they are implementing. 
© East African Community 2011 — All rights reserved * 
East Africa
n Community  
P O Box1096  
Arusha  
Tanzania  Tel: 255 27 2504253/8 
Fax: 255-27-2504481/2504255 
E-Mail: eac@eachq.org
 
Web: www.each.org  
 
 
                                                      
* © 2011  EAC — All rights of exploitation in any form and by an y means reserved worldwide for EAC Partner States’ NSBs.  
 EAS 753:2011 
© EAC 2011 – All rights reserved                 iii 
 Acknowledgement 
This standard was devel oped with support from the Policy Analysi s and Advocacy Programme (PAAP) of the 
Association for Strengthening Agricult ural Research in Eastern and Cent ral Africa (ASARECA). This was 
possible though a grant by the United States Agency for International Development (USAID). This support 
was used in the process of formulation and mobilization of stakeholders to review the standard in national and 
regional fora. 
 ASARECA is a non-political association of agricultural research institutes  in: Burundi, DR Congo, Eritrea, 
Ethiopia, Kenya, Madagascar, Rwanda, Sudan, Tanzani a and Uganda. ASARECA serves  as a platform for 
promoting regional research and in the sharing of benef its and spillovers that derive from such research. The 
mission of ASARECA is to “ Enhance regional collective action in agricultural research for development, 
extension and agricultural training and education, to pr omote economic growth, fight poverty, eradicate hunger 
and enhance sustainable use of resources in Eastern and Central Africa ”. 
 Development of standards has been part of PAAP’s contribution to changing the way business is done in 
crucial agricultural sectors to increase effici ency and/or reduce waste through rationalization and 
harmonization of policies, laws, regulations and proc edures. Rationalization focuses on how countries conduct 
business in a given subsector, and determines what should be done to make the procedures and processes more efficient. Harmonization brings together regionally  different approaches (policies, laws, regulations and 
procedures) into unified approaches that are applied acro ss the countries. This harmonization process allows 
commodities and factors to move freely across national boundaries, thereby improving domestic and foreign investment by expanding markets beyond national borders. Over time this will lead to gradual attainment of 
seamless borders for trade in potato and  potato products across the region. 
 
Removal of regulatory bottlenecks to trans-boundary movement of  seed potato in the region will enhance 
competitiveness of trade and value addition in the sub-se ctor. It will improve the value chains by supporting 
product differentiation and hence increased trade in seed pot ato in the region. This will ultimately contribute to 
improved incomes, employment generation and welfare in t he region. This fits snugly with the aspirations of 
ASARECA as a key player contributing to economic development of the region. EAST AFRICAN STANDARD EAS 753:2011
 
1               © EAC 2011 – All rights reserved 
 Seed potato— Specification 
1 Scope 
This East African Standard specifies require ments and methods of sampling and test for seed 
potato. It specifie
s requirements for varietal i dentity, purity; genealogy, traceability, pests and 
diseases, internal and external quality, physiology, sizing, packaging and labelling. 
2 Normative references 
The following referenced documents are indispensable for the application of this document. For dated 
references, only the edition cited applies. For undated  references, the latest edition of the referenced 
document (including any amendments) applies.  
National   seed certification legislations EAC Plant Protection Legislation  
3 Terms and definitions 
For the purpose of this standard, the fo llowing terms and definitions shall apply. 
3.1 
blackleg: 
Commonly used name of a bacterial disease of potatoes, caused by Erwinia carotovora  subsp. 
atroseptica . Similar symptoms may, however, be caused by E. carotovora subsp. carotovora and 
E.chrysanthemi. 
3.2 
certification: 
An official control procedure which aims at ensu ring the production and supply of seed potato which 
satisfy the requirements of this standard. 
3.3 
consignment: 
a quantity of seed potato consisting of one or more lo ts destined to a party and  covered by one set of 
documents. 
3.4 
contaminated field:  a field whose history and regulatory action indicates presence of a designated pathogenic organism in 
the soil. 
3.5 
disease: 
any disturbance of a plant caused by pathogenic organis ms which interferes with  its normal structure, 
physiological function or economic value. EAS 753:2011 9 
 
© EAC 2011 – All rights reserved               2 
 3.6 
field: 
a defined area of land used for cultivation of seed potato. 
3.7 
practically free from: 
not present  in numbers or quantities that can be detected by the application of appropriate sampling, 
inspection and testing procedures. 
3.8 
generation number  the number of growing cycles since the first introduction in the field.  
3.9  
homogeneous uniform in composition and appearance. 
3.10 
initial Stock the initial pathogen free  microplants or tubers  produced and maintained under an official control 
programme. 
3.11 
inspection 
examination of plants, tubers, un its of presentation, equipment or facilities by National Designated 
Authority, to determine compliance with regulatory   requirements.
 
3.12 
lot 
a quantity of seed potato bearing the same reference number, and being of the same variety, 
category, class, size and origin. 
3.13 
National Designated Authority (NDA): organization, or agency empowered by national legislation to administer the certification of seed 
potato under the Standard. 
3.14 
origin 
officially defined area where a lo t of seed potato was  grown.  
3.15  
phytosanitary measures 
provisions in accordance with the National or Regional  Plant Protection legislation and/or 
International Plant Protection Convention. 
3.16  
primary virus infection: 
attack occurring during the current growing season and not arising from the seed tuber. 
3.17  
quality: 
the sum of all characteristics that determine the acceptance of seed potatoes in relation to the 
specifications of this Standard. EAS 753:2011 
© EAC 2011 – All rights reserved               3 
 
 3.18 
quality Control: 
the control by the NDA of all activities encountered  in the process of producing and marketing seed 
potato in conformance with the Standard. 
3.19 
pest 
Any species, strain or biotype of plant, animal or pathogenic agent injurious to plants or plant 
products.  
3.20 
quarantine pest 
A pest of potential national economic importanc e to the country endangered thereby and not yet 
present there,or present but not widely dist ributed and being officially  controlled. 
3.21 
regulated non-quarantine pest 
A non-quarantine pest whose presence in plants for planting affects the intended use of these plants 
with an economically unacceptable impact and which is therefore regulated with the territory of the 
importing ‘contracting’ part. 
3.22 
sampling 
The procedure of drawing at random a number of tubers, plants or parts of plants, which may be 
taken as representative of the lot or the field. 
3.23 
seed potato 
tubers  of Solanum tuberosum  L which are certified by the NDA. 
3.24 
sprout inhibitor chemical substance, applied either to the plants during the growing season or to the tubers after 
harvest which suppresses the nor mal development of sprouts. 
4 Provisions for variety 
Varieties shall be accepted into the Standard only if it has been officially released by the NDA. 
5   General requirements 
Seed potato shall be practically free from injurious diseases and pests and from any defects likely to 
impair their quality as seed.  
Seed potato shall be substantially dry outside and,  in general, of normal shape for the variety.  
Seed potato or growing crops of potato shall not be  treated with sprout inhibitors.  Neither growing 
crops of seed potato nor seed potato shall be treated with sprout inhibitors without permission from 
the NDA.  EAS 753:2011 9 
 
© EAC 2011 – All rights reserved               4 
 6  Classifications 
Seed potato shall be classified according to variety,  standards and generations.  Classification shall 
be subject to official control in the producing c ountry. Seed potato shall be placed in three classes as  
defined in Table 1:   
Table 1: Seed Potato  Classes 
Code  Classes  Seed Parents Colour of labels  
SS Breeders Seed Progeny of parental stock  White with a diagonal 
purple stripe 
B. Basic Progeny of certifi ed breeders seed or certified 
pre-basic seed White 
C.1 Cer. 1st gen. Progeny of certified pre-basic seed or certified 
basic seed Blue 
C.2 Cert. 2nd Gen. Progeny of certified basic seed or certified 1st 
generation Blue 
C.3 Cert. 3rd gen. 
(Standard) Progeny of certified 2nd generation or certified 3rd 
generation Blue 
 
6.1 Breeders seed 
Progeny of parental stock. The breede rs seed may include pre-basic seed.  
6.2 Basic Seed 
Seed potato descended directly fr om breeders seed or produced un der special provisions of a 
national certification scheme and ar e mainly intended for the producti on of certified seed potatoes.  
6.3 Certified Seed 
Seed potato descended directly from breeder. , Basic or Certified seed and are mainly intended for 
the production of potato other than seed potato. Seed  shall be classified as Certified I, Certified II or 
Certified III according to the minimu m requirements given in sections 7.2, 7.3 and 7.4 
7 Requirements for seed potato  
7.1 General requirements 
7.1.1 The parent material shall be true to type for the variety. 
7.1.2 Seed potato shall be produced from officially cert ified initial stock, which shall be practically free 
from the current most limiting pests and diseases including the following; 
a) Potato spindle tuber viroid 
b) Clavibacter michiganensis spp. sepedonicus (ring rot) 
c) Ralstonia solanacearum (Bacterial wilt)  
d) Erwinia spp. EAS 753:2011 
© EAC 2011 – All rights reserved               5 
 
 e) Potato viruses X, Y, S, M and A 
f) Potato Leafroll Virus 
g) Late brite 
h) Nematodes  i) Synchytrium endobioticum (Schilb or Wart disease) 
7.1.3 The facilities and proc edures used for seed potat o production shall be subject to official 
approval by the NDA.  
7.2 Requirements for the field 
Field for the production of seed potato shall meet the requirements specified under table 2. 
Table 2: Field requirements 
 Seed Classes 
 Basic C1 C2 C3 
Land requirements (seasons 
free of potato or related family)  5 3* 3* 3* 
Isolation (meters)  10 5 5 5 
Maximum off-types % BR=0 
PB=1 
B=2 2/100 plants 2/100 plants 3/100 
plants 
Minimum number of 
inspections  4 4 4 4 
*for Bacterial Wilt, 7 year rotation is required. 
7.3 Requirements during storage  
During storage, seed potato shall conform to the tolerance limits ac ceptable for pests and disease as 
specified under table 3;   
 
 
  
 
 EAS 753:2011 9 
 
© EAC 2011 – All rights reserved               6 
 Table 3: Tolerance levels - pests and diseases during storage 
 Seed Classes 
 Basic C1 C2 C3 
Scab % max (No more than 
50%tuber covered) per 25Kg bag  25 50 50 50 
Rhizoctonia %  10 30 30 30 
Pink rot ( Phytophthora 
erythroseptica) %  0 1 1 1 
Soft rot ( Erwinia spp ) %  0 1 1 1 
Severe Tuber Moth %  2 5 5 5 
 
7.4 Quality requirements for seed potato 
7.4.1 General  
The seed potato shall be free from Globodera rostochiensis  (Woll), Globodera pallida  (Stone), 
Synchytrium endobioticum  (Schilb.) Prc,  Heterodera  rostochiensis, Clavibacter michiganensis  Spp. 
sepedonicus  (Spieck. And Kotth.) Skapt. and Burkh., Ralstonia solanacearum  (E.F. Smith) E.F. 
Smith, Potato spindle tuber viroid, Tomato St olbur and Meloidogyne chitwoodi  mycoplasma and 
fallax. 
7.4.2 Tolerances for pests and disease 
Toleran
ce limits   for pest and disease allowed for seed potato tubers shall be as specified under table 
4; 
Table 4: Tolerance limits for pests and disease 
 Seed Classes 
 Basic C1 C2 C3 
Mycoplasma  1/1000 plants  1/1000 plants  1/1000 plants  1/1000 plants 
Potato Virus Y  1/1000 plants  10/1000 plants  13/1000 plants  13/1000 plants 
SMV  0 13/1000 plants  15/1000 plants  15/1000 plants 
Fusarium wilt  0  2/1000 plants  3/1000 plants  3/1000 plants 
Verticilium wilt  0 2/1000 plants  3/1000 plants  4/1000 plants 
 EAS 753:2011 
© EAC 2011 – All rights reserved               7 
 
 7.4.3 Tolerance requirements for a lot 
During inspection, the allowed tolerance limits   for a lot   shall be as specified under table 5 below; 
Table 5: Tolerance levels for defects and diseases of seed potato 
S/N Requirement Breeders Basic seed Certified seed 
1 Presence of earth and extraneous matter (%by 
weight)  1% 2% 2% 
2 Dry and wet rot 0% 1% 1% 
3 External defects (e.g. malformed  or damaged 
tubers) 3% 3% 3% 
4 Common scab tubers affected over a specified 
percent of their surface 
Max by weight 0%surface cover 33.3% surface cover 33.3% surface cover 
0 5 5 
5 Powdery scab 5: tubers affected over a 
specified percent of their surface Max % by weight 0%surface cover 10% surface cover 10% surface cover 
0 3 3 
6 Rhizoctonia tubers affected over a specified per 
cent of their surface % by weight max 0%surface cover 10% surface cover 10% surface cover 
0 5 5 
7 Shrivelled tubers which have become 
excessively dehydrated and wrinkled. % by weight 0 1 1 
8 Requirements for sizing 
8.1 General requirements 
Breeder seed are exempted from the minimum sizing requirements. 
Sizing shall be as specified under   Table 6.  
Table 6: Size requirements 
Seed Classes 
Size Basic  C1  C2  C3 
28-45 mm  5 tubers/25kg bag  5 tubers/25kg bag  5 tubers/25kg bag  5 tubers/25kg bag 
46 – 55 mm  5 tubers/25kg bag  5 tubers/25kg bag  5 tubers/ 25kg bag  5 tubers/25kg bag 
9  Packaging 
9.1  Presentations EAS 753:2011 9 
 
© EAC 2011 – All rights reserved               8 
 9.1.1 Condition of units of presentation  
The package shall not be more than 50kg, shall be clean and allow for aeration and which conforms 
to the regulations in the destination country. The reuse of packages shall not be allowed.  
 
9.1.2 Sealing of units of presentation 
The corresponding NDA shall supervise the sealing of  containers. In case of need, re-sealing of the 
package or container shall be done under the official control by the NDA. 
10 Labelling 
10.1 Official label 
Units of presentation shall bear an official label in accordance with 10.2 and which has not been 
previously used; the label shall be white with a di agonal purple line for breeder seed white for basic 
seed and blue for certified seed. 
10.2 Required information the official label  
10.2.1 . Name of the product shall be "Seed potato" 
10.2.2 . The National Designated Authority (NDA) or its recognized initials 
10.2.3 . Origin of the product 
10.2.4 . Official lot number  
10.2.5 . Month and year of harvesting   
10.2.6 . Date of packaging 
10.2.7 . Variety 
10.2.8 . Class  
10.2.9 . Tuber size 
10.2.10. Declared average net weight at packaging   
10.2.11 . Warning “Not for human consumption”  
 
10.3 Additional information 
Without contracting the information on the official label, the producer may provide additional 
information and such information may not be misleading.  EAS 753:2011 
© EAC 2011 – All rights reserved               9 
 
 10.4 Chemical treatment 
The nature of the active substance of any chemical  treatment of the seed potato shall be indicated 
either on the outside of the unit of presentation, on th e official label or a label provided by the supplier, 
or printed on the unit of presentation. This information may also appear inside the unit of presentation. 
10.5 Re-inspection and re-labelling 
If re-inspection is conducted, the authority which carried out the re-inspecti on shall be stated on the 
new label, as well as the date of the re-sealing.   Re-labelling shall be done under the supervision of 
the NDA. The new label shall show the particulars which appeared on the old label. If a new label is 
necessary, this shall show the partic ulars, which appeared on the old label, the date of the re-closing 
and the authority concerned. 
11 Sampling 
Sampling of seed potato for certification purposes shall be carried in accordance with Annex A. 
12 Comparative tests 
It is recommended that trials be established by the NDA  to ascertain the conditi on of the certified seed 
potato. The NDA shall provide the test guidelines to be followed. The results of such trials shall be 
treated in confidence but on request the results relating to individual consignments may be 
exchanged between the NDA of the impor ting and exporting countries concerned. 
13 Compliance 
 A seed potato lot shall be deemed to comply  if upon inspection and testing, the provisions under this 
standard are met.  EAS 753:2011 9 
 
© EAC 2011 – All rights reserved               10 
 Annex A 
(Normative) 
A.  Organizing the inspection of crops grow n from sample lots of seed potato 
(certified according to this  standard) 
A.1. Purpose of the inspection 
The examination of seed potato in crop tests en ables the quality (vigour, purity, healthiness, 
productivity) of home grown and imported lots  put on the market to be checked at random. 
A.2. Organization 
A.2.1. Place of sampling 
Depending on the mode of transport (road, rail or waterway), the sample shall preferably be taken 
when the lot arrives at its destination. 
A.2.2. Organs responsible for the sampling 
The sampling shall be done by the NDA. 
A.2.3. Sampling 
(a) The lot as defined in section 2.14 is the unit repr esented by a sample. If the lot is a large one, the 
number of samples shall be increased to: 
i) One sample per wagon or vehicle, in the case of transport by rail or road, 
ii) One sample for every 50 tonnes, in the case of transport by ship. (b) A sample consists of 110 tubers, taken from different places in the container or from at least 10 
sacks. 
(c) The sample shall be placed in a sealed sack; its label shall bear the number of the wagon or the 
name of the ship. 
A.2.4. Preservation of samples 
Samples shall be preserved in a uniform manner in favourable conditions. 
A.2.5. Trial fields 
(a) The land shall be suitable for potato growing. 
(b) Planting shall be done in plots of 100 plants. The plots shall be grouped by variety in order to 
facilitate comparison. EAS 753:2011 
© EAC 2011 – All rights reserved               11 
 
 (c) Manuring shall be adapted to the needs of the crop, but moderate; the use of nitrogen during 
growth shall be prohibited.  
(d) The usual cultural care shall be conducive to keeping the field clean and the foliage intact 
A.2.6. List of plots 
A nomenclature of all the samples planted in the sa me field with the number of the plot concerned 
shall be sent to the organs responsible for evaluating them. 
A.2.7. Evaluation of the crop inspection 
In order to be accurate, the evaluation shall in principl e be carried out in two stages, with an interval of 
10-15 days between them. Primary virus infectio ns shall not be taken into consideration. EAS 753:2011 9 
 
© EAC 2011 – All rights reserved               12 
 Annex B  
 (Informative)  
 EAS 753:2011 
© EAC 2011 – All rights reserved               13 
 
 
 
 EAS 753:2011 9 
 
© EAC 2011 – All rights reserved               14 
 Annex C 
 (Informative) 
C.1 Sampling tubers for virus testing 
In testing seed stocks for the incidence of virus, it is  seldom feasible to test the entire stock, so a test 
is done on a sample from the stock. Ideally, only seed  stocks with infection levels below the tolerance 
would be accepted and those above the tolerance rejected. However, taking a sample from a stock 
means that only estimation of the actual incidenc e of virus can be made. The reliability of this 
estimation will vary with the size of the sample, relative to the size of the lot, and the Population standard, which is set for the test. Defining an acceptable population standard for any sample entails 
two types of risk. 
The first is that of rejecting a stock containing less virus than the tolerance and is often described as 
the A grower = s at risk. The risk of accepting a stock containing more virus than the tolerance is 
known as the Abuyer = s at risk. 
 From the point of view of classification authorities, this could also be described as the risk of passing 
a stock which fails to meet the official tolerances. Such testing makes a number of important 
assumptions, which are, primarily, that the infe cted tubers are distributed hom ogeneously in the stock 
and that tubers are sampled randomly. In addition, the choice of the size of sample to be tested will 
need to be balanced by other practical factors such as  cost, available facilities, labour, logistics of 
handling samples, seed stock size, etc. The following tables and graphs illustrate some of the 
principles involved in sampling tubers for testing for virus. 
C.2 Confidence limits 
Testing different samples from the same seed stock will give a range of results, which, statistically, 
will lie within a specific interval with a certain percentage confidence. This interval is known as the 
confidence interval. The acceptable level of confidence or probability should be decided before the 
testing is conducted but 95% confid ence/probability is normally used. The accuracy of the estimation 
can be improved by increasing the sample size and by adjusting the allowable number of infected tubers in the sample, i.e. the sample tolerance. (Table C.1). 
For example, the size of the confidence interval fo r a sample tolerance of 4% (4 allowable tubers) is 
8.8% based on a sample of 100 tubers but, on a sample  of 200 tubers, the interval decreases to 6% 
i.e. 7.7-1.7. The effect on the confidence interval of increasing  the sample size does, however, 
become smaller at the larger sample sizes. Incr easing the sample size from 100 to 200 tubers 
improves the accuracy of the estimation by 32 %, i. e. confidence interval re duced from 8.8 to 6.0%, 
whereas increasing the sample size from 300 to 400 tubers only gives an improvement of 15%. EAS 753:2011 
© EAC 2011 – All rights reserved               15 
 
 In practice, therefore, the benefits of increasing the sample size have to be weighed up against the 
additional cost of the testing. The accuracy of the estimation can also be affected by changing the 
allowable number of infected tubers in the sample  (table 4). For example, by decreasing the number 
of allowable tubers from 4 to 3, i. e. changing sample tolerance from 4 to 3 %, the confidence interval 
is decreased from 8.8 to 7.9 % and the confidence limits themselves become lower. Decreasing the 
allowable number of infected tubers in the sample also has a significant effect on the probability of 
classifying at higher tolerances than those allowed in the sample as illustrated in the next paragraph. 
Table C.1: Confidence limits, at a probability of 95%, for various sample tolerances of virus in 
relation to the size of the sample. 
Tolerance (%) for 
virus in a seed stock Size of sample Allowable No of 
infected tubers Confidence limits 
   Lower  Upper  
4 100 4 (3) 1.1(0.6)  9.9(8.5)  
 200 8(7)  1.7(1.4)  7.7(7.1)  
 300 12(11)  2.1(1.8)  6.9(6.5)  
 400 16(15)  2.3(2.1)  6.4(6.1)  
10 100 10(8)  4.9(3.5)  17.6(15.2)  
 200 20(18)  6.2(5.4)  15.0(14.0)  
 300 30  6.9 13.8 
 400 40 7.2 13.8 
 
Probability of classifying stocks to  meet specified tolerances 
From the confidence intervals, it can be seen that classifying stocks based on a sample will contain a 
risk that some stocks, which fail a test, do, in fac t, meet the tolerance and others, which pass, should 
fail.  
Table C.2 and Figure 1 show the effe ct of varying sample size and the number of virus infected tubers 
allowed in the sample onthe probability of lassifying seed stocks wi th different incidences of virus 
infection. For example, in a test on a sample of 100 tubers where 3 virus infected tubers were 
allowed, there would be a 14% chance of classify ing a stock containing 6% virus as meeting a 
tolerance of 4%. Probability of classifying seed  stocks at two tolerances for virus based on a 
laboratory test in relation to the size of sample and the allowable number of virus-infected tubers in 
the sample: 
 EAS 753:2011 9 
 
© EAC 2011 – All rights reserved               16 
 Table C.2 
 
 
Note : The allowable number of tubers is, often, set at a lowe r level than the overall seed stock tolerance of 4 and 10% 
respectively, particularly in the case of a relatively sma ll sample size. By lowering the tolerance in a Sample the > 
buyers risk = is reduced. 
 
 
Figure 1: Probability of classifying seed stocks with different incidences of virus as meeting a tolerance of 4% or 10% 
for virus in a laboratory test in relation to the size of sample and the allowable number of virus infected tubers in the 
sample; 
 tolerance up to 4% 
tolerance up to 10% EAS 753:2011 
© EAC 2011 – All rights reserved               17 
 
 
 
 
 EAS 753:2011  
  
© EAC 2011 — All rights reserved