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Estimates and standard error of the estimates

The results published in the NFI tables are estimates for variables in the Swiss forest (population parameters) whose true values are not known and must therefore be extrapolated (estimated) from the NFI sample data.

The extrapolations carry uncertainties. However, the accuracy of the extrapolations can be reliably estimated from the NFI sample, which is designed to be a random sample. To this end, all NFI tables include a second figure in addition to the estimate itself, namely the standard error of the estimate.

In most tables the percent standard error is reported (±%), but the absolute standard error (±) is also reported occasionally, especially for estimated percentages. The relationship between absolute and percent standard error is as follows:

percent standard error = absolute standard error / estimate × 100

absolute standard error = percent standard error × estimate / 100

Information on the forest road network is based on a full survey of all forest roads. In this case, there is no need to specify a standard error because there is no uncertainty caused by sampling.

Confidence interval of the estimate

With the estimate and the standard error of the estimate, the limits of the so-called confidence interval of the estimate can be calculated as:

lower limit = estimate – tQ × absolute standard error

upper limit = estimate + tQ × absolute standard error

If the simple standard error is used for the calculation (tQ = 1), then the 68% confidence interval is formed. In this case, the true value of the population parameter has a 68% probability of lying within this confidence interval of the estimate. If the double standard error is used (tQ = 2), then there is a 95% probability that the true value lies within this 95% confidence interval.

Significance of the estimate

The confidence interval can be used to statistically test whether the estimated population parameter is larger or smaller than a specific reference or target value, or whether two estimated population parameters truly differ (in the real population). For practical applications, proceed as follows: if a reference value lies outside the confidence interval, the estimated population value can be interpreted as being significantly different from it; if it lies within the confidence interval, the difference between the estimated population parameter and the reference value can be interpreted as random or not significant. When two population parameters are compared, they can be interpreted as differing significantly if their confidence intervals do not overlap.

Dealing with missing values

When calculating a results table, data are not always available for all combinations of characteristics^* of the classification variables and regional demarcation applied. In most cases, this indicates that the attribute estimated with the respective target variable does not occur or occurs only very rarely. Usually, the value 0 is used (imputed). However, as this value is not based on direct measurements, the associated standard error is represented with a full stop [.]. If the imputed value of 0 is referred to in the calculation, e.g. in the case of percentages or certain change estimates, no value can be entered. In this case, the estimated value and standard error are both represented with a full stop [.].

For example, no Arolla pines have been found and measured in the Swiss Plateau to date (growing stock of Arolla pines by production region). The values can be assumed to be missing because Arolla pine does not occur in the Swiss Plateau and therefore the growing stock there must be 0.

^* e.g. ‘Arolla pine’ characteristic of the classification variable ‘tree species’ and ‘Swiss plateau’ characteristic of the ‘regional demarcation’

Changes

There are two types of changes in the NFI:

The first type involves specific target variables relating to components of change, such as increment, fellings, mortality and losses. These target variables are only available for two consecutive inventory cycles, e.g. NFI3–NFI4. In the evaluation of components of change, the classification variable characteristic of the second inventory cycle is assigned to the first inventory cycle. These evaluations therefore do not take into account potential changes in a classification variable characteristic from the earlier to the later inventory (e.g. from private to public ownership).

With the second type of change, the difference in target variables, such as number of stems, growing stock or forest area, is used to assess the change between two inventory cycles. Target variables are generally used to represent states, such as that in NFI4, but can also indicate the net change between any two inventory cycles, e.g. NFI1–NFI4. In evaluations of change using these target variables, the change in a classification variable characteristic is considered in the analysis. Consequently, one can detect, for example, that the forest area without shrub forest increased between the two inventory cycles. This only has an effect on classification variables whose characteristic (class) can actually change, e.g. the affiliation with a forest area or the tree condition.

NFI1
number of stems
diameter at breast height (in 4 cm classes)
regional demarcation: biogeographical region
unit: 1000 n
evaluation area: accessible forest without shrub forest
grid: 1.4-km grid
state 1983/85
	biogeographical region
	Jura		Plateau		Northern Alps		Western Central-Alps		Eastern Central-Alps		Southern Alps		Switzerland
diameter at breast height (in 4 cm classes)	1000 n	±%	1000 n	±%	1000 n	±%	1000 n	±%	1000 n	±%	1000 n	±%	1000 n	±%
n/a	0	.	0	.	0	.	0	.	0	.	0	.	0	.
12-15 cm	18698	4	26853	4	32384	4	9720	7	11009	7	17290	5	115953	2
16-19 cm	14320	4	17863	4	24578	4	6900	8	7870	7	11942	5	83474	2
20-23 cm	10797	4	14287	4	17657	4	5103	8	6462	7	7131	6	61437	2
24-27 cm	9531	4	11600	4	14876	4	3876	8	5346	7	4969	6	50197	2
28-31 cm	8606	4	10017	4	11753	4	3342	8	4451	7	3563	7	41733	2
32-35 cm	6512	5	9071	4	10512	4	2432	9	3961	7	2339	8	34828	2
36-39 cm	4951	4	7942	3	8034	3	1657	8	3084	6	1762	7	27430	2
40-43 cm	3891	4	6255	4	6705	4	1467	8	2486	6	1267	8	22071	2
44-47 cm	2886	5	4520	4	5266	4	988	9	1992	6	1023	8	16675	2
48-51 cm	1906	5	3335	4	3846	4	734	10	1522	7	901	8	12245	2
52-55 cm	1204	7	2095	5	2617	5	604	11	1034	8	495	10	8049	3
56-59 cm	894	8	1543	6	1968	6	418	12	785	9	476	10	6083	3
60-63 cm	377	11	760	8	939	7	213	15	400	11	242	14	2932	4
64-67 cm	298	12	604	9	735	9	107	20	281	13	187	18	2213	5
68-71 cm	172	17	325	14	485	10	110	22	251	14	154	18	1497	6
72-75 cm	114	21	180	16	291	13	92	22	156	18	116	21	950	7
76-79 cm	32	35	120	20	178	18	53	30	68	29	123	23	574	10
80-83 cm	33	35	114	20	106	20	28	38	39	32	76	28	397	11
84-87 cm	9	71	27	41	53	28	12	58	27	38	36	36	163	16
88-91 cm	12	58	31	35	32	35	16	50	8	71	20	45	120	18
92-95 cm	4	100	12	58	20	45	4	100	4	100	19	52	62	26
96-99 cm	4	100	12	58	24	41	8	71	21	45	28	43	97	21
100-103 cm	0	.	4	100	7	71	0	0	4	100	23	45	37	34
104-107 cm	9	72	0	.	17	50	4	100	8	71	22	46	60	27
108-111 cm	0	.	0	.	5	100	4	100	0	.	0	.	9	71
112-115 cm	0	.	0	.	0	.	0	.	0	.	4	100	4	100
116-119 cm	0	.	0	.	0	.	0	.	0	.	7	100	7	100
120-123 cm	0	.	0	.	0	.	0	.	0	.	6	100	6	100
124-127 cm	0	.	0	.	0	.	0	.	0	.	8	71	8	71
128-131 cm	0	.	0	.	0	.	0	.	0	.	4	100	4	100
132-135 cm	0	.	0	.	0	.	0	.	0	.	0	.	0	.
136-139 cm	0	.	0	.	0	.	0	.	0	.	0	.	0	.
140-143 cm	0	.	0	.	0	.	0	.	0	.	0	.	0	.
144-147 cm	0	.	0	.	0	.	0	.	0	.	4	100	4	100
148-151 cm	0	.	0	.	0	.	0	.	0	.	4	100	4	100
152-155 cm	0	.	0	.	0	.	0	.	0	.	4	100	4	100
156-159 cm	0	.	0	.	0	.	0	.	0	.	8	100	8	100
160-163 cm	0	.	0	.	0	.	0	.	0	.	0	.	0	.
164-167 cm	0	.	0	.	0	.	0	.	0	.	4	100	4	100
168-171 cm	0	.	0	.	0	.	0	.	0	.	0	.	0	.
172-175 cm	0	.	0	.	0	.	0	.	0	.	0	.	0	.
176-179 cm	0	.	0	.	0	.	0	.	0	.	0	.	0	.
180-183 cm	0	.	0	.	0	.	0	.	0	.	0	.	0	.
184-187 cm	0	.	0	.	0	.	0	.	0	.	4	100	4	100
188-191 cm	0	.	0	.	0	.	0	.	0	.	0	.	0	.
192-195 cm	0	.	0	.	0	.	0	.	0	.	4	100	4	100
196-199 cm	0	.	0	.	0	.	0	.	0	.	4	100	4	100
=200 cm	0	.	0	.	0	.	0	.	0	.	0	.	0	.
total	85257	3	117570	2	143090	2	37893	5	51270	4	54268	3	489347	1

Table citation

Abegg, M.; Ahles, P.; Allgaier Leuch, B.; Cioldi, F.; Didion, M.; Düggelin, C.; Fischer, C.; Herold, A.; Meile, R.; Rohner, B.; Rösler, E.; Speich, S.; Temperli, C.; Traub, B.,
2023: Swiss national forest inventory - Result table No. 1239348. Birmensdorf, Swiss Federal Research Institute WSL

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